diff --git a/CLAUDE.md b/CLAUDE.md
index bfc4958..0eba926 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -191,6 +191,15 @@ The app has an interactive tutorial system (`static/js/features/tutorials.js`) w
 
 When adding **new tabs, sections, or major UI elements**, update the corresponding tutorial step array in `tutorials.js` and add `tour.*` i18n keys to all 3 locale files (`en.json`, `ru.json`, `zh.json`).
 
+## Localization (i18n)
+
+**Every user-facing string must be localized.** Never use hardcoded English strings in `showToast()`, `error.textContent`, modal messages, or any other UI-visible text. Always use `t('key')` from `../core/i18n.js` and add the corresponding key to **all three** locale files (`en.json`, `ru.json`, `zh.json`).
+
+- In JS modules: `import { t } from '../core/i18n.js';` then `showToast(t('my.key'), 'error')`
+- In inline `<script>` blocks (where `t()` may not be available yet): use `window.t ? t('key') : 'fallback'`
+- In HTML templates: use `data-i18n="key"` for text content, `data-i18n-title="key"` for title attributes, `data-i18n-aria-label="key"` for aria-labels
+- Keys follow dotted namespace convention: `feature.context.description` (e.g. `device.error.brightness`, `calibration.saved`)
+
 ## General Guidelines
 
 - Always test changes before marking as complete
diff --git a/server/src/wled_controller/api/routes/picture_targets.py b/server/src/wled_controller/api/routes/picture_targets.py
index 9907ce8..8b1d9c2 100644
--- a/server/src/wled_controller/api/routes/picture_targets.py
+++ b/server/src/wled_controller/api/routes/picture_targets.py
@@ -1,5 +1,6 @@
 """Picture target routes: CRUD, processing control, settings, state, metrics."""
 
+import asyncio
 import base64
 import io
 import secrets
@@ -297,9 +298,10 @@ async def update_target(
             if "brightness_value_source_id" in kc_incoming:
                 kc_brightness_vs_changed = True
 
-        # Sync processor manager
+        # Sync processor manager (run in thread — css release/acquire can block)
         try:
-            target.sync_with_manager(
+            await asyncio.to_thread(
+                target.sync_with_manager,
                 manager,
                 settings_changed=(data.fps is not None or
                                   data.keepalive_interval is not None or
diff --git a/server/src/wled_controller/api/routes/system.py b/server/src/wled_controller/api/routes/system.py
index e9350f1..b088f0b 100644
--- a/server/src/wled_controller/api/routes/system.py
+++ b/server/src/wled_controller/api/routes/system.py
@@ -1,5 +1,6 @@
 """System routes: health, version, displays, performance, backup/restore, ADB."""
 
+import asyncio
 import io
 import json
 import platform
@@ -372,15 +373,20 @@ async def restore_config(
         if not isinstance(stores[key], dict):
             raise HTTPException(status_code=400, detail=f"Store '{key}' in backup is not a valid JSON object")
 
-    # Write store files atomically
+    # Write store files atomically (in thread to avoid blocking event loop)
     config = get_config()
-    written = 0
-    for store_key, config_attr in STORE_MAP.items():
-        if store_key in stores:
-            file_path = Path(getattr(config.storage, config_attr))
-            atomic_write_json(file_path, stores[store_key])
-            written += 1
-            logger.info(f"Restored store: {store_key} -> {file_path}")
+
+    def _write_stores():
+        count = 0
+        for store_key, config_attr in STORE_MAP.items():
+            if store_key in stores:
+                file_path = Path(getattr(config.storage, config_attr))
+                atomic_write_json(file_path, stores[store_key])
+                count += 1
+                logger.info(f"Restored store: {store_key} -> {file_path}")
+        return count
+
+    written = await asyncio.to_thread(_write_stores)
 
     logger.info(f"Restore complete: {written}/{len(STORE_MAP)} stores written. Scheduling restart...")
     _schedule_restart()
diff --git a/server/src/wled_controller/core/audio/analysis.py b/server/src/wled_controller/core/audio/analysis.py
index 9b25abb..5af0354 100644
--- a/server/src/wled_controller/core/audio/analysis.py
+++ b/server/src/wled_controller/core/audio/analysis.py
@@ -99,6 +99,12 @@ class AudioAnalyzer:
         self._spectrum_buf_right = np.zeros(NUM_BANDS, dtype=np.float32)
         self._sq_buf = np.empty(chunk_size, dtype=np.float32)
 
+        # Pre-compute band start/end arrays and widths for vectorized binning
+        self._band_starts = np.array([s for s, _ in self._bands], dtype=np.intp)
+        self._band_ends = np.array([e for _, e in self._bands], dtype=np.intp)
+        self._band_widths = (self._band_ends - self._band_starts).astype(np.float32)
+        self._band_widths[self._band_widths == 0] = 1.0  # avoid divide-by-zero
+
         # Pre-allocated channel buffers for stereo
         self._left_buf = np.empty(chunk_size, dtype=np.float32)
         self._right_buf = np.empty(chunk_size, dtype=np.float32)
@@ -205,11 +211,15 @@ class AudioAnalyzer:
         fft_mag = np.abs(np.fft.rfft(self._fft_windowed))
         fft_mag *= (1.0 / chunk_size)
         fft_len = len(fft_mag)
-        for b, (s, e) in enumerate(self._bands):
-            if s < fft_len and e <= fft_len:
-                buf[b] = float(np.mean(fft_mag[s:e]))
-            else:
-                buf[b] = 0.0
+        # Vectorized band binning using cumulative sum
+        valid = (self._band_starts < fft_len) & (self._band_ends <= fft_len) & (self._band_ends > 0)
+        buf[:] = 0.0
+        if valid.any():
+            cumsum = np.cumsum(fft_mag)
+            band_sums = cumsum[self._band_ends[valid] - 1] - np.where(
+                self._band_starts[valid] > 0, cumsum[self._band_starts[valid] - 1], 0.0
+            )
+            buf[valid] = band_sums / self._band_widths[valid]
         spec_max = float(np.max(buf))
         if spec_max > 1e-6:
             buf *= (1.0 / spec_max)
diff --git a/server/src/wled_controller/core/capture/pixel_processor.py b/server/src/wled_controller/core/capture/pixel_processor.py
index cde905c..97cdff0 100644
--- a/server/src/wled_controller/core/capture/pixel_processor.py
+++ b/server/src/wled_controller/core/capture/pixel_processor.py
@@ -1,18 +1,27 @@
 """Pixel processing utilities for color correction and manipulation."""
 
-from typing import List, Tuple
+from typing import List, Tuple, Union
 import numpy as np
 
 from wled_controller.utils import get_logger
 
 logger = get_logger(__name__)
 
+ColorList = Union[List[Tuple[int, int, int]], np.ndarray]
+
+
+def _as_array(colors: ColorList) -> np.ndarray:
+    """Convert list-of-tuples to (N,3) uint8 array, or pass through if already ndarray."""
+    if isinstance(colors, np.ndarray):
+        return colors
+    return np.array(colors, dtype=np.uint8)
+
 
 def smooth_colors(
-    current_colors: List[Tuple[int, int, int]],
-    previous_colors: List[Tuple[int, int, int]],
+    current_colors: ColorList,
+    previous_colors: ColorList,
     smoothing_factor: float = 0.5,
-) -> List[Tuple[int, int, int]]:
+) -> np.ndarray:
     """Smooth color transitions between frames.
 
     Args:
@@ -21,96 +30,71 @@ def smooth_colors(
         smoothing_factor: Smoothing amount (0.0-1.0, where 0=no smoothing, 1=full smoothing)
 
     Returns:
-        Smoothed colors
+        Smoothed colors as (N,3) uint8 ndarray
     """
-    if not current_colors or not previous_colors:
-        return current_colors
+    if not len(current_colors) or not len(previous_colors):
+        return _as_array(current_colors)
 
     if len(current_colors) != len(previous_colors):
         logger.warning(
             f"Color count mismatch: current={len(current_colors)}, "
             f"previous={len(previous_colors)}. Skipping smoothing."
         )
-        return current_colors
+        return _as_array(current_colors)
 
     if smoothing_factor <= 0:
-        return current_colors
+        return _as_array(current_colors)
     if smoothing_factor >= 1:
-        return previous_colors
+        return _as_array(previous_colors)
 
-    # Convert to numpy arrays
-    current = np.array(current_colors, dtype=np.float32)
-    previous = np.array(previous_colors, dtype=np.float32)
-
-    # Blend between current and previous
+    current = np.asarray(current_colors, dtype=np.float32)
+    previous = np.asarray(previous_colors, dtype=np.float32)
     smoothed = current * (1 - smoothing_factor) + previous * smoothing_factor
-
-    # Convert back to integers
-    smoothed = np.clip(smoothed, 0, 255).astype(np.uint8)
-
-    return [tuple(color) for color in smoothed]
+    return np.clip(smoothed, 0, 255).astype(np.uint8)
 
 
 def adjust_brightness_global(
-    colors: List[Tuple[int, int, int]],
+    colors: ColorList,
     target_brightness: int,
-) -> List[Tuple[int, int, int]]:
+) -> np.ndarray:
     """Adjust colors to achieve target global brightness.
 
     Args:
-        colors: List of (R, G, B) tuples
+        colors: List of (R, G, B) tuples or (N,3) ndarray
         target_brightness: Target brightness (0-255)
 
     Returns:
-        Adjusted colors
+        Adjusted colors as (N,3) uint8 ndarray
     """
-    if not colors or target_brightness == 255:
-        return colors
+    arr = _as_array(colors)
+    if not len(arr) or target_brightness == 255:
+        return arr
 
-    # Calculate scaling factor
     scale = target_brightness / 255.0
-
-    # Scale all colors
-    scaled = [
-        (
-            int(r * scale),
-            int(g * scale),
-            int(b * scale),
-        )
-        for r, g, b in colors
-    ]
-
-    return scaled
+    return (arr.astype(np.float32) * scale).astype(np.uint8)
 
 
 def limit_brightness(
-    colors: List[Tuple[int, int, int]],
+    colors: ColorList,
     max_brightness: int = 255,
-) -> List[Tuple[int, int, int]]:
+) -> np.ndarray:
     """Limit maximum brightness of any color channel.
 
     Args:
-        colors: List of (R, G, B) tuples
+        colors: List of (R, G, B) tuples or (N,3) ndarray
         max_brightness: Maximum allowed brightness (0-255)
 
     Returns:
-        Limited colors
+        Limited colors as (N,3) uint8 ndarray
     """
-    if not colors or max_brightness == 255:
-        return colors
+    arr = _as_array(colors)
+    if not len(arr) or max_brightness == 255:
+        return arr
 
-    limited = []
-    for r, g, b in colors:
-        # Find max channel value
-        max_val = max(r, g, b)
-
-        if max_val > max_brightness:
-            # Scale down proportionally
-            scale = max_brightness / max_val
-            r = int(r * scale)
-            g = int(g * scale)
-            b = int(b * scale)
-
-        limited.append((r, g, b))
-
-    return limited
+    arr_f = arr.astype(np.float32)
+    max_vals = np.max(arr_f, axis=1)
+    need_scale = max_vals > max_brightness
+    if need_scale.any():
+        scales = np.where(need_scale, max_brightness / np.maximum(max_vals, 1.0), 1.0)
+        arr_f *= scales[:, np.newaxis]
+    return arr_f.astype(np.uint8)
diff --git a/server/src/wled_controller/core/devices/wled_client.py b/server/src/wled_controller/core/devices/wled_client.py
index 08d7036..d925bc9 100644
--- a/server/src/wled_controller/core/devices/wled_client.py
+++ b/server/src/wled_controller/core/devices/wled_client.py
@@ -404,8 +404,13 @@ class WLEDClient(LEDClient):
         """
         try:
             # Build indexed pixel array: [led_index, r, g, b, ...]
-            indices = np.arange(len(pixels), dtype=np.int32).reshape(-1, 1)
-            indexed_pixels = np.hstack([indices, pixels.astype(np.int32)]).ravel().tolist()
+            n = len(pixels)
+            flat = np.empty(n * 4, dtype=np.int32)
+            flat[0::4] = np.arange(n, dtype=np.int32)
+            flat[1::4] = pixels[:, 0]
+            flat[2::4] = pixels[:, 1]
+            flat[3::4] = pixels[:, 2]
+            indexed_pixels = flat.tolist()
 
             # Build WLED JSON state
             payload = {
diff --git a/server/src/wled_controller/core/devices/wled_provider.py b/server/src/wled_controller/core/devices/wled_provider.py
index 0ff7fff..98dce1b 100644
--- a/server/src/wled_controller/core/devices/wled_provider.py
+++ b/server/src/wled_controller/core/devices/wled_provider.py
@@ -24,6 +24,25 @@ DEFAULT_SCAN_TIMEOUT = 3.0
 class WLEDDeviceProvider(LEDDeviceProvider):
     """Provider for WLED LED controllers."""
 
+    def __init__(self):
+        self._http_client: Optional[httpx.AsyncClient] = None
+        self._client_lock = asyncio.Lock()
+
+    async def _get_client(self) -> httpx.AsyncClient:
+        """Return a shared HTTP client (connection-pooled, thread-safe init)."""
+        if self._http_client is not None and not self._http_client.is_closed:
+            return self._http_client
+        async with self._client_lock:
+            if self._http_client is None or self._http_client.is_closed:
+                self._http_client = httpx.AsyncClient(timeout=5.0)
+        return self._http_client
+
+    async def close(self):
+        """Close the shared HTTP client."""
+        if self._http_client is not None and not self._http_client.is_closed:
+            await self._http_client.aclose()
+            self._http_client = None
+
     @property
     def device_type(self) -> str:
         return "wled"
@@ -158,46 +177,46 @@ class WLEDDeviceProvider(LEDDeviceProvider):
 
     async def get_brightness(self, url: str) -> int:
         url = url.rstrip("/")
-        async with httpx.AsyncClient(timeout=5.0) as http_client:
-            resp = await http_client.get(f"{url}/json/state")
-            resp.raise_for_status()
-            state = resp.json()
-            return state.get("bri", 255)
+        client = await self._get_client()
+        resp = await client.get(f"{url}/json/state")
+        resp.raise_for_status()
+        state = resp.json()
+        return state.get("bri", 255)
 
     async def set_brightness(self, url: str, brightness: int) -> None:
         url = url.rstrip("/")
-        async with httpx.AsyncClient(timeout=5.0) as http_client:
-            resp = await http_client.post(
-                f"{url}/json/state",
-                json={"bri": brightness},
-            )
-            resp.raise_for_status()
+        client = await self._get_client()
+        resp = await client.post(
+            f"{url}/json/state",
+            json={"bri": brightness},
+        )
+        resp.raise_for_status()
 
     async def get_power(self, url: str, **kwargs) -> bool:
         url = url.rstrip("/")
-        async with httpx.AsyncClient(timeout=5.0) as http_client:
-            resp = await http_client.get(f"{url}/json/state")
-            resp.raise_for_status()
-            return resp.json().get("on", False)
+        client = await self._get_client()
+        resp = await client.get(f"{url}/json/state")
+        resp.raise_for_status()
+        return resp.json().get("on", False)
 
     async def set_power(self, url: str, on: bool, **kwargs) -> None:
         url = url.rstrip("/")
-        async with httpx.AsyncClient(timeout=5.0) as http_client:
-            resp = await http_client.post(
-                f"{url}/json/state",
-                json={"on": on},
-            )
-            resp.raise_for_status()
+        client = await self._get_client()
+        resp = await client.post(
+            f"{url}/json/state",
+            json={"on": on},
+        )
+        resp.raise_for_status()
 
     async def set_color(self, url: str, color: Tuple[int, int, int], **kwargs) -> None:
         """Set WLED to a solid color using the native segment color API."""
         url = url.rstrip("/")
-        async with httpx.AsyncClient(timeout=5.0) as http_client:
-            resp = await http_client.post(
-                f"{url}/json/state",
-                json={
-                    "on": True,
-                    "seg": [{"col": [[color[0], color[1], color[2]]], "fx": 0}],
-                },
-            )
-            resp.raise_for_status()
+        client = await self._get_client()
+        resp = await client.post(
+            f"{url}/json/state",
+            json={
+                "on": True,
+                "seg": [{"col": [[color[0], color[1], color[2]]], "fx": 0}],
+            },
+        )
+        resp.raise_for_status()
diff --git a/server/src/wled_controller/core/processing/effect_stream.py b/server/src/wled_controller/core/processing/effect_stream.py
index 859c73e..0d454b7 100644
--- a/server/src/wled_controller/core/processing/effect_stream.py
+++ b/server/src/wled_controller/core/processing/effect_stream.py
@@ -356,12 +356,17 @@ class EffectColorStripStream(ColorStripStream):
             new_heat[-1] = heat[-1] * 0.5
             heat[:] = new_heat
 
-        # Sparks at the bottom
+        # Sparks at the bottom (vectorized)
         spark_zone = max(1, n // 8)
         spark_prob = 0.3 * intensity
-        for i in range(spark_zone):
-            if np.random.random() < spark_prob:
-                heat[i] = min(1.0, heat[i] + 0.4 + 0.6 * np.random.random())
+        rng = np.random.random(spark_zone)
+        mask = rng < spark_prob
+        if mask.any():
+            heat[:spark_zone] = np.where(
+                mask,
+                np.minimum(1.0, heat[:spark_zone] + 0.4 + 0.6 * np.random.random(spark_zone)),
+                heat[:spark_zone],
+            )
 
         # Map heat to palette (pre-allocated scratch)
         np.multiply(heat, 255, out=self._s_f32_a)
diff --git a/server/src/wled_controller/core/processing/kc_target_processor.py b/server/src/wled_controller/core/processing/kc_target_processor.py
index c5fbee0..2d98313 100644
--- a/server/src/wled_controller/core/processing/kc_target_processor.py
+++ b/server/src/wled_controller/core/processing/kc_target_processor.py
@@ -174,8 +174,8 @@ class KCTargetProcessor(TargetProcessor):
         self._latest_colors = None
 
         # Start processing task
-        self._task = asyncio.create_task(self._processing_loop())
         self._is_running = True
+        self._task = asyncio.create_task(self._processing_loop())
 
         logger.info(f"Started KC processing for target {self._target_id}")
         self._ctx.fire_event({"type": "state_change", "target_id": self._target_id, "processing": True})
diff --git a/server/src/wled_controller/core/processing/mapped_stream.py b/server/src/wled_controller/core/processing/mapped_stream.py
index 4c4935f..a442cb0 100644
--- a/server/src/wled_controller/core/processing/mapped_stream.py
+++ b/server/src/wled_controller/core/processing/mapped_stream.py
@@ -152,10 +152,10 @@ class MappedColorStripStream(ColorStripStream):
     # ── Processing loop ─────────────────────────────────────────
 
     def _processing_loop(self) -> None:
+        frame_time = 1.0 / self._fps
         try:
             while self._running:
                 loop_start = time.perf_counter()
-                frame_time = 1.0 / self._fps
 
                 try:
                     target_n = self._led_count
diff --git a/server/src/wled_controller/core/processing/wled_target_processor.py b/server/src/wled_controller/core/processing/wled_target_processor.py
index 5d7e8f7..1392ebb 100644
--- a/server/src/wled_controller/core/processing/wled_target_processor.py
+++ b/server/src/wled_controller/core/processing/wled_target_processor.py
@@ -160,8 +160,8 @@ class WledTargetProcessor(TargetProcessor):
 
         # Reset metrics and start loop
         self._metrics = ProcessingMetrics(start_time=datetime.utcnow())
-        self._task = asyncio.create_task(self._processing_loop())
         self._is_running = True
+        self._task = asyncio.create_task(self._processing_loop())
 
         logger.info(f"Started processing for target {self._target_id}")
         self._ctx.fire_event({"type": "state_change", "target_id": self._target_id, "processing": True})
@@ -889,6 +889,10 @@ class WledTargetProcessor(TargetProcessor):
                 await _probe_client.aclose()
             if _probe_task is not None and not _probe_task.done():
                 _probe_task.cancel()
+                try:
+                    await _probe_task
+                except (asyncio.CancelledError, Exception):
+                    pass
             self._device_reachable = None
             self._metrics.device_streaming_reachable = None
             logger.info(f"Processing loop ended for target {self._target_id}")
diff --git a/server/src/wled_controller/static/js/features/audio-sources.js b/server/src/wled_controller/static/js/features/audio-sources.js
index fe2fcd0..0d77362 100644
--- a/server/src/wled_controller/static/js/features/audio-sources.js
+++ b/server/src/wled_controller/static/js/features/audio-sources.js
@@ -148,10 +148,11 @@ export async function saveAudioSource() {
 export async function editAudioSource(sourceId) {
     try {
         const resp = await fetchWithAuth(`/audio-sources/${sourceId}`);
-        if (!resp.ok) throw new Error('fetch failed');
+        if (!resp.ok) throw new Error(t('audio_source.error.load'));
         const data = await resp.json();
         await showAudioSourceModal(data.source_type, data);
     } catch (e) {
+        if (e.isAuth) return;
         showToast(e.message, 'error');
     }
 }
@@ -161,7 +162,7 @@ export async function editAudioSource(sourceId) {
 export async function cloneAudioSource(sourceId) {
     try {
         const resp = await fetchWithAuth(`/audio-sources/${sourceId}`);
-        if (!resp.ok) throw new Error('fetch failed');
+        if (!resp.ok) throw new Error(t('audio_source.error.load'));
         const data = await resp.json();
         delete data.id;
         data.name = data.name + ' (copy)';
diff --git a/server/src/wled_controller/static/js/features/calibration.js b/server/src/wled_controller/static/js/features/calibration.js
index 1936333..91b5262 100644
--- a/server/src/wled_controller/static/js/features/calibration.js
+++ b/server/src/wled_controller/static/js/features/calibration.js
@@ -6,6 +6,7 @@ import {
     calibrationTestState, EDGE_TEST_COLORS,
 } from '../core/state.js';
 import { API_BASE, getHeaders, fetchWithAuth } from '../core/api.js';
+import { t } from '../core/i18n.js';
 import { showToast } from '../core/ui.js';
 import { Modal } from '../core/modal.js';
 import { closeTutorial, startCalibrationTutorial } from './tutorials.js';
@@ -138,7 +139,7 @@ export async function showCalibration(deviceId) {
             fetchWithAuth('/config/displays'),
         ]);
 
-        if (!response.ok) { showToast('Failed to load calibration', 'error'); return; }
+        if (!response.ok) { showToast(t('calibration.error.load_failed'), 'error'); return; }
 
         const device = await response.json();
         const calibration = device.calibration;
@@ -215,7 +216,7 @@ export async function showCalibration(deviceId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to load calibration:', error);
-        showToast('Failed to load calibration', 'error');
+        showToast(t('calibration.error.load_failed'), 'error');
     }
 }
 
@@ -240,7 +241,7 @@ export async function showCSSCalibration(cssId) {
             fetchWithAuth('/devices'),
         ]);
 
-        if (!cssResp.ok) { showToast('Failed to load color strip source', 'error'); return; }
+        if (!cssResp.ok) { showToast(t('calibration.error.css_load_failed'), 'error'); return; }
         const source = await cssResp.json();
         const calibration = source.calibration || {};
 
@@ -339,7 +340,7 @@ export async function showCSSCalibration(cssId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to load CSS calibration:', error);
-        showToast('Failed to load calibration', 'error');
+        showToast(t('calibration.error.load_failed'), 'error');
     }
 }
 
@@ -841,13 +842,13 @@ export async function toggleTestEdge(edge) {
             });
             if (!response.ok) {
                 const errorData = await response.json();
-                error.textContent = `Test failed: ${errorData.detail}`;
+                error.textContent = t('calibration.error.test_toggle_failed');
                 error.style.display = 'block';
             }
         } catch (err) {
             if (err.isAuth) return;
             console.error('Failed to toggle CSS test edge:', err);
-            error.textContent = 'Failed to toggle test edge';
+            error.textContent = t('calibration.error.test_toggle_failed');
             error.style.display = 'block';
         }
         return;
@@ -871,13 +872,13 @@ export async function toggleTestEdge(edge) {
         });
         if (!response.ok) {
             const errorData = await response.json();
-            error.textContent = `Test failed: ${errorData.detail}`;
+            error.textContent = t('calibration.error.test_toggle_failed');
             error.style.display = 'block';
         }
     } catch (err) {
         if (err.isAuth) return;
         console.error('Failed to toggle test edge:', err);
-        error.textContent = 'Failed to toggle test edge';
+        error.textContent = t('calibration.error.test_toggle_failed');
         error.style.display = 'block';
     }
 }
@@ -920,13 +921,13 @@ export async function saveCalibration() {
         : parseInt(document.getElementById('cal-device-led-count-inline').textContent) || 0;
     if (!cssMode) {
         if (total !== declaredLedCount) {
-            error.textContent = `Total LEDs (${total}) must equal device LED count (${declaredLedCount})`;
+            error.textContent = t('calibration.error.led_count_mismatch');
             error.style.display = 'block';
             return;
         }
     } else {
         if (declaredLedCount > 0 && total > declaredLedCount) {
-            error.textContent = `Calibrated LEDs (${total}) exceed total LED count (${declaredLedCount})`;
+            error.textContent = t('calibration.error.led_count_exceeded');
             error.style.display = 'block';
             return;
         }
@@ -963,7 +964,7 @@ export async function saveCalibration() {
             });
         }
         if (response.ok) {
-            showToast('Calibration saved', 'success');
+            showToast(t('calibration.saved'), 'success');
             calibModal.forceClose();
             if (cssMode) {
                 if (window.loadTargetsTab) window.loadTargetsTab();
@@ -972,13 +973,13 @@ export async function saveCalibration() {
             }
         } else {
             const errorData = await response.json();
-            error.textContent = `Failed to save: ${errorData.detail}`;
+            error.textContent = t('calibration.error.save_failed');
             error.style.display = 'block';
         }
     } catch (err) {
         if (err.isAuth) return;
         console.error('Failed to save calibration:', err);
-        error.textContent = 'Failed to save calibration';
+        error.textContent = t('calibration.error.save_failed');
         error.style.display = 'block';
     }
 }
diff --git a/server/src/wled_controller/static/js/features/color-strips.js b/server/src/wled_controller/static/js/features/color-strips.js
index 8bf69de..b7870ac 100644
--- a/server/src/wled_controller/static/js/features/color-strips.js
+++ b/server/src/wled_controller/static/js/features/color-strips.js
@@ -525,7 +525,7 @@ async function _loadAudioSources() {
     if (!select) return;
     try {
         const resp = await fetchWithAuth('/audio-sources');
-        if (!resp.ok) throw new Error('fetch failed');
+        if (!resp.ok) return;
         const data = await resp.json();
         const sources = data.sources || [];
         select.innerHTML = sources.map(s => {
@@ -905,7 +905,7 @@ export async function showCSSEditor(cssId = null, cloneData = null) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to open CSS editor:', error);
-        showToast('Failed to open color strip editor', 'error');
+        showToast(t('color_strip.error.editor_open_failed'), 'error');
     }
 }
 
@@ -1097,7 +1097,7 @@ export async function cloneColorStrip(cssId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone color strip:', error);
-        showToast('Failed to clone color strip source', 'error');
+        showToast(t('color_strip.error.clone_failed'), 'error');
     }
 }
 
@@ -1122,7 +1122,7 @@ export async function deleteColorStrip(cssId) {
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to delete color strip source', 'error');
+        showToast(t('color_strip.error.delete_failed'), 'error');
     }
 }
 
diff --git a/server/src/wled_controller/static/js/features/dashboard.js b/server/src/wled_controller/static/js/features/dashboard.js
index c62c523..b887294 100644
--- a/server/src/wled_controller/static/js/features/dashboard.js
+++ b/server/src/wled_controller/static/js/features/dashboard.js
@@ -198,10 +198,16 @@ function _updateRunningMetrics(enrichedRunning) {
         if (chart) {
             const actualH = _fpsHistory[target.id] || [];
             const currentH = _fpsCurrentHistory[target.id] || [];
-            chart.data.datasets[0].data = [...actualH];
-            chart.data.datasets[1].data = [...currentH];
-            chart.data.labels = actualH.map(() => '');
-            chart.update();
+            // Mutate in-place to avoid array copies
+            const ds0 = chart.data.datasets[0].data;
+            ds0.length = 0;
+            ds0.push(...actualH);
+            const ds1 = chart.data.datasets[1].data;
+            ds1.length = 0;
+            ds1.push(...currentH);
+            while (chart.data.labels.length < ds0.length) chart.data.labels.push('');
+            chart.data.labels.length = ds0.length;
+            chart.update('none');
         }
 
         // Refresh uptime base for interpolation
@@ -366,11 +372,14 @@ export async function loadDashboard(forceFullRender = false) {
     setTabRefreshing('dashboard-content', true);
 
     try {
-        const [targetsResp, profilesResp, devicesResp, cssResp] = await Promise.all([
+        // Fire all requests in a single batch to avoid sequential RTTs
+        const [targetsResp, profilesResp, devicesResp, cssResp, batchStatesResp, batchMetricsResp] = await Promise.all([
             fetchWithAuth('/picture-targets'),
             fetchWithAuth('/profiles').catch(() => null),
             fetchWithAuth('/devices').catch(() => null),
             fetchWithAuth('/color-strip-sources').catch(() => null),
+            fetchWithAuth('/picture-targets/batch/states').catch(() => null),
+            fetchWithAuth('/picture-targets/batch/metrics').catch(() => null),
         ]);
 
         const targetsData = await targetsResp.json();
@@ -384,6 +393,9 @@ export async function loadDashboard(forceFullRender = false) {
         const cssSourceMap = {};
         for (const s of (cssData.sources || [])) { cssSourceMap[s.id] = s; }
 
+        const allStates = batchStatesResp && batchStatesResp.ok ? (await batchStatesResp.json()).states : {};
+        const allMetrics = batchMetricsResp && batchMetricsResp.ok ? (await batchMetricsResp.json()).metrics : {};
+
         // Build dynamic HTML (targets, profiles)
         let dynamicHtml = '';
         let runningIds = [];
@@ -392,12 +404,6 @@ export async function loadDashboard(forceFullRender = false) {
         if (targets.length === 0 && profiles.length === 0) {
             dynamicHtml = `<div class="dashboard-no-targets">${t('dashboard.no_targets')}</div>`;
         } else {
-            const [batchStatesResp, batchMetricsResp] = await Promise.all([
-                fetchWithAuth('/picture-targets/batch/states'),
-                fetchWithAuth('/picture-targets/batch/metrics'),
-            ]);
-            const allStates = batchStatesResp.ok ? (await batchStatesResp.json()).states : {};
-            const allMetrics = batchMetricsResp.ok ? (await batchMetricsResp.json()).metrics : {};
             const enriched = targets.map(target => ({
                 ...target,
                 state: allStates[target.id] || {},
@@ -712,7 +718,7 @@ export async function dashboardToggleProfile(profileId, enable) {
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to toggle profile', 'error');
+        showToast(t('dashboard.error.profile_toggle_failed'), 'error');
     }
 }
 
@@ -726,11 +732,11 @@ export async function dashboardStartTarget(targetId) {
             loadDashboard();
         } else {
             const error = await response.json();
-            showToast(`Failed to start: ${error.detail}`, 'error');
+            showToast(t('dashboard.error.start_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to start processing', 'error');
+        showToast(t('dashboard.error.start_failed'), 'error');
     }
 }
 
@@ -744,11 +750,11 @@ export async function dashboardStopTarget(targetId) {
             loadDashboard();
         } else {
             const error = await response.json();
-            showToast(`Failed to stop: ${error.detail}`, 'error');
+            showToast(t('dashboard.error.stop_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to stop processing', 'error');
+        showToast(t('dashboard.error.stop_failed'), 'error');
     }
 }
 
@@ -763,26 +769,31 @@ export async function dashboardToggleAutoStart(targetId, enable) {
             loadDashboard();
         } else {
             const error = await response.json();
-            showToast(`Failed: ${error.detail}`, 'error');
+            showToast(t('dashboard.error.autostart_toggle_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to toggle auto-start', 'error');
+        showToast(t('dashboard.error.autostart_toggle_failed'), 'error');
     }
 }
 
 export async function dashboardStopAll() {
     try {
-        const targetsResp = await fetchWithAuth('/picture-targets');
+        const [targetsResp, statesResp] = await Promise.all([
+            fetchWithAuth('/picture-targets'),
+            fetchWithAuth('/picture-targets/batch/states'),
+        ]);
         const data = await targetsResp.json();
-        const running = (data.targets || []).filter(t => t.id);
+        const statesData = statesResp.ok ? await statesResp.json() : { states: {} };
+        const states = statesData.states || {};
+        const running = (data.targets || []).filter(t => states[t.id]?.processing);
         await Promise.all(running.map(t =>
             fetchWithAuth(`/picture-targets/${t.id}/stop`, { method: 'POST' }).catch(() => {})
         ));
         loadDashboard();
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to stop all targets', 'error');
+        showToast(t('dashboard.error.stop_all'), 'error');
     }
 }
 
diff --git a/server/src/wled_controller/static/js/features/device-discovery.js b/server/src/wled_controller/static/js/features/device-discovery.js
index 4202d01..4313957 100644
--- a/server/src/wled_controller/static/js/features/device-discovery.js
+++ b/server/src/wled_controller/static/js/features/device-discovery.js
@@ -317,7 +317,7 @@ export async function handleAddDevice(event) {
     }
 
     if (!name || (!isMockDevice(deviceType) && !url)) {
-        error.textContent = 'Please fill in all fields';
+        error.textContent = t('device_discovery.error.fill_all_fields');
         error.style.display = 'block';
         return;
     }
@@ -351,7 +351,7 @@ export async function handleAddDevice(event) {
         if (response.ok) {
             const result = await response.json();
             console.log('Device added successfully:', result);
-            showToast('Device added successfully', 'success');
+            showToast(t('device_discovery.added'), 'success');
             addDeviceModal.forceClose();
             // Use window.* to avoid circular imports
             if (typeof window.loadDevices === 'function') await window.loadDevices();
@@ -365,12 +365,12 @@ export async function handleAddDevice(event) {
         } else {
             const errorData = await response.json();
             console.error('Failed to add device:', errorData);
-            error.textContent = `Failed to add device: ${errorData.detail}`;
+            error.textContent = t('device_discovery.error.add_failed');
             error.style.display = 'block';
         }
     } catch (err) {
         if (err.isAuth) return;
         console.error('Failed to add device:', err);
-        showToast('Failed to add device', 'error');
+        showToast(t('device_discovery.error.add_failed'), 'error');
     }
 }
diff --git a/server/src/wled_controller/static/js/features/devices.js b/server/src/wled_controller/static/js/features/devices.js
index 35f316c..d831675 100644
--- a/server/src/wled_controller/static/js/features/devices.js
+++ b/server/src/wled_controller/static/js/features/devices.js
@@ -126,7 +126,7 @@ export async function turnOffDevice(deviceId) {
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to turn off device', 'error');
+        showToast(t('device.error.power_off_failed'), 'error');
     }
 }
 
@@ -143,23 +143,23 @@ export async function removeDevice(deviceId) {
             method: 'DELETE',
         });
         if (response.ok) {
-            showToast('Device removed', 'success');
+            showToast(t('device.removed'), 'success');
             window.loadDevices();
         } else {
             const error = await response.json();
-            showToast(`Failed to remove: ${error.detail}`, 'error');
+            showToast(t('device.error.remove_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to remove device:', error);
-        showToast('Failed to remove device', 'error');
+        showToast(t('device.error.remove_failed'), 'error');
     }
 }
 
 export async function showSettings(deviceId) {
     try {
         const deviceResponse = await fetchWithAuth(`/devices/${deviceId}`);
-        if (!deviceResponse.ok) { showToast('Failed to load device settings', 'error'); return; }
+        if (!deviceResponse.ok) { showToast(t('device.error.settings_load_failed'), 'error'); return; }
 
         const device = await deviceResponse.json();
         const isAdalight = isSerialDevice(device.device_type);
@@ -171,7 +171,7 @@ export async function showSettings(deviceId) {
 
         document.getElementById('settings-device-id').value = device.id;
         document.getElementById('settings-device-name').value = device.name;
-        document.getElementById('settings-health-interval').value = 30;
+        document.getElementById('settings-health-interval').value = device.state_check_interval ?? 30;
 
         const isMock = isMockDevice(device.device_type);
         const urlGroup = document.getElementById('settings-url-group');
@@ -242,7 +242,7 @@ export async function showSettings(deviceId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to load device settings:', error);
-        showToast('Failed to load device settings', 'error');
+        showToast(t('device.error.settings_load_failed'), 'error');
     }
 }
 
@@ -256,12 +256,16 @@ export async function saveDeviceSettings() {
     const url = settingsModal._getUrl();
 
     if (!name || !url) {
-        settingsModal.showError('Please fill in all fields correctly');
+        settingsModal.showError(t('device.error.required'));
         return;
     }
 
     try {
-        const body = { name, url, auto_shutdown: document.getElementById('settings-auto-shutdown').checked };
+        const body = {
+            name, url,
+            auto_shutdown: document.getElementById('settings-auto-shutdown').checked,
+            state_check_interval: parseInt(document.getElementById('settings-health-interval').value, 10) || 30,
+        };
         const ledCountInput = document.getElementById('settings-led-count');
         if (settingsModal.capabilities.includes('manual_led_count') && ledCountInput.value) {
             body.led_count = parseInt(ledCountInput.value, 10);
@@ -283,7 +287,7 @@ export async function saveDeviceSettings() {
 
         if (!deviceResponse.ok) {
             const errorData = await deviceResponse.json();
-            settingsModal.showError(`Failed to update device: ${errorData.detail}`);
+            settingsModal.showError(t('device.error.update'));
             return;
         }
 
@@ -293,7 +297,7 @@ export async function saveDeviceSettings() {
     } catch (err) {
         if (err.isAuth) return;
         console.error('Failed to save device settings:', err);
-        settingsModal.showError('Failed to save settings');
+        settingsModal.showError(t('device.error.save'));
     }
 }
 
@@ -307,14 +311,16 @@ export async function saveCardBrightness(deviceId, value) {
     const bri = parseInt(value);
     updateDeviceBrightness(deviceId, bri);
     try {
-        await fetch(`${API_BASE}/devices/${deviceId}/brightness`, {
+        const resp = await fetchWithAuth(`/devices/${deviceId}/brightness`, {
             method: 'PUT',
-            headers: getHeaders(),
             body: JSON.stringify({ brightness: bri })
         });
+        if (!resp.ok) {
+            showToast(t('device.error.brightness'), 'error');
+        }
     } catch (err) {
-        console.error('Failed to update brightness:', err);
-        showToast('Failed to update brightness', 'error');
+        if (err.isAuth) return;
+        showToast(t('device.error.brightness'), 'error');
     }
 }
 
@@ -323,9 +329,7 @@ export async function fetchDeviceBrightness(deviceId) {
     if (_brightnessFetchInFlight.has(deviceId)) return;
     _brightnessFetchInFlight.add(deviceId);
     try {
-        const resp = await fetch(`${API_BASE}/devices/${deviceId}/brightness`, {
-            headers: getHeaders()
-        });
+        const resp = await fetchWithAuth(`/devices/${deviceId}/brightness`);
         if (!resp.ok) return;
         const data = await resp.json();
         updateDeviceBrightness(deviceId, data.brightness);
@@ -398,9 +402,7 @@ async function _populateSettingsSerialPorts(currentUrl) {
 
     try {
         const discoverType = settingsModal.deviceType || 'adalight';
-        const resp = await fetch(`${API_BASE}/devices/discover?timeout=2&device_type=${encodeURIComponent(discoverType)}`, {
-            headers: getHeaders()
-        });
+        const resp = await fetchWithAuth(`/devices/discover?timeout=2&device_type=${encodeURIComponent(discoverType)}`);
         if (!resp.ok) return;
         const data = await resp.json();
         const devices = data.devices || [];
diff --git a/server/src/wled_controller/static/js/features/kc-targets.js b/server/src/wled_controller/static/js/features/kc-targets.js
index c537a28..e3900eb 100644
--- a/server/src/wled_controller/static/js/features/kc-targets.js
+++ b/server/src/wled_controller/static/js/features/kc-targets.js
@@ -505,7 +505,7 @@ export async function showKCEditor(targetId = null, cloneData = null) {
         setTimeout(() => document.getElementById('kc-editor-name').focus(), 100);
     } catch (error) {
         console.error('Failed to open KC editor:', error);
-        showToast('Failed to open key colors editor', 'error');
+        showToast(t('kc_target.error.editor_open_failed'), 'error');
     }
 }
 
@@ -588,13 +588,13 @@ export async function saveKCEditor() {
 
 export async function cloneKCTarget(targetId) {
     try {
-        const resp = await fetch(`${API_BASE}/picture-targets/${targetId}`, { headers: getHeaders() });
+        const resp = await fetchWithAuth(`/picture-targets/${targetId}`);
         if (!resp.ok) throw new Error('Failed to load target');
         const target = await resp.json();
         showKCEditor(null, target);
     } catch (error) {
-        console.error('Failed to clone KC target:', error);
-        showToast('Failed to clone key colors target', 'error');
+        if (error.isAuth) return;
+        showToast(t('kc_target.error.clone_failed'), 'error');
     }
 }
 
@@ -613,11 +613,11 @@ export async function deleteKCTarget(targetId) {
             if (typeof window.loadTargetsTab === 'function') window.loadTargetsTab();
         } else {
             const error = await response.json();
-            showToast(`Failed to delete: ${error.detail}`, 'error');
+            showToast(t('kc_target.error.delete_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to delete key colors target', 'error');
+        showToast(t('kc_target.error.delete_failed'), 'error');
     }
 }
 
diff --git a/server/src/wled_controller/static/js/features/pattern-templates.js b/server/src/wled_controller/static/js/features/pattern-templates.js
index e224805..0f7b271 100644
--- a/server/src/wled_controller/static/js/features/pattern-templates.js
+++ b/server/src/wled_controller/static/js/features/pattern-templates.js
@@ -125,7 +125,7 @@ export async function showPatternTemplateEditor(templateId = null, cloneData = n
         setTimeout(() => document.getElementById('pattern-template-name').focus(), 100);
     } catch (error) {
         console.error('Failed to open pattern template editor:', error);
-        showToast('Failed to open pattern template editor', 'error');
+        showToast(t('pattern.error.editor_open_failed'), 'error');
     }
 }
 
@@ -189,13 +189,13 @@ export async function savePatternTemplate() {
 
 export async function clonePatternTemplate(templateId) {
     try {
-        const resp = await fetch(`${API_BASE}/pattern-templates/${templateId}`, { headers: getHeaders() });
+        const resp = await fetchWithAuth(`/pattern-templates/${templateId}`);
         if (!resp.ok) throw new Error('Failed to load pattern template');
         const tmpl = await resp.json();
         showPatternTemplateEditor(null, tmpl);
     } catch (error) {
-        console.error('Failed to clone pattern template:', error);
-        showToast('Failed to clone pattern template', 'error');
+        if (error.isAuth) return;
+        showToast(t('pattern.error.clone_failed'), 'error');
     }
 }
 
@@ -216,11 +216,11 @@ export async function deletePatternTemplate(templateId) {
             if (typeof window.loadTargetsTab === 'function') window.loadTargetsTab();
         } else {
             const error = await response.json();
-            showToast(`Failed to delete: ${error.detail}`, 'error');
+            showToast(t('pattern.error.delete_failed'), 'error');
         }
     } catch (error) {
         if (error.isAuth) return;
-        showToast('Failed to delete pattern template', 'error');
+        showToast(t('pattern.error.delete_failed'), 'error');
     }
 }
 
@@ -825,6 +825,6 @@ export async function capturePatternBackground() {
         }
     } catch (error) {
         console.error('Failed to capture background:', error);
-        showToast('Failed to capture background', 'error');
+        showToast(t('pattern.error.capture_bg_failed'), 'error');
     }
 }
diff --git a/server/src/wled_controller/static/js/features/perf-charts.js b/server/src/wled_controller/static/js/features/perf-charts.js
index 6bede60..b5f6775 100644
--- a/server/src/wled_controller/static/js/features/perf-charts.js
+++ b/server/src/wled_controller/static/js/features/perf-charts.js
@@ -144,9 +144,13 @@ function _pushSample(key, value) {
     if (_history[key].length > MAX_SAMPLES) _history[key].shift();
     const chart = _charts[key];
     if (!chart) return;
-    chart.data.datasets[0].data = [..._history[key]];
-    chart.data.labels = _history[key].map(() => '');
-    chart.update();
+    const ds = chart.data.datasets[0].data;
+    ds.length = 0;
+    ds.push(..._history[key]);
+    // Ensure labels array matches length (reuse existing array)
+    while (chart.data.labels.length < ds.length) chart.data.labels.push('');
+    chart.data.labels.length = ds.length;
+    chart.update('none');
 }
 
 async function _fetchPerformance() {
diff --git a/server/src/wled_controller/static/js/features/streams.js b/server/src/wled_controller/static/js/features/streams.js
index 4cfd10a..8756ee5 100644
--- a/server/src/wled_controller/static/js/features/streams.js
+++ b/server/src/wled_controller/static/js/features/streams.js
@@ -242,19 +242,26 @@ function restoreCaptureDuration() {
 }
 
 export async function showTestTemplateModal(templateId) {
-    const templates = await fetchWithAuth('/capture-templates').then(r => r.json());
-    const template = templates.templates.find(t => t.id === templateId);
+    try {
+        const resp = await fetchWithAuth('/capture-templates');
+        if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
+        const data = await resp.json();
+        const template = (data.templates || []).find(tp => tp.id === templateId);
 
-    if (!template) {
+        if (!template) {
+            showToast(t('templates.error.load'), 'error');
+            return;
+        }
+
+        window.currentTestingTemplate = template;
+        await loadDisplaysForTest();
+        restoreCaptureDuration();
+
+        testTemplateModal.open();
+    } catch (error) {
+        if (error.isAuth) return;
         showToast(t('templates.error.load'), 'error');
-        return;
     }
-
-    window.currentTestingTemplate = template;
-    await loadDisplaysForTest();
-    restoreCaptureDuration();
-
-    testTemplateModal.open();
 }
 
 export function closeTestTemplateModal() {
@@ -871,7 +878,7 @@ export async function cloneAudioTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone audio template:', error);
-        showToast('Failed to clone audio template', 'error');
+        showToast(t('audio_template.error.clone_failed'), 'error');
     }
 }
 
@@ -2213,7 +2220,7 @@ export async function cloneStream(streamId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone stream:', error);
-        showToast('Failed to clone picture source', 'error');
+        showToast(t('stream.error.clone_picture_failed'), 'error');
     }
 }
 
@@ -2226,7 +2233,7 @@ export async function cloneCaptureTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone capture template:', error);
-        showToast('Failed to clone capture template', 'error');
+        showToast(t('stream.error.clone_capture_failed'), 'error');
     }
 }
 
@@ -2239,7 +2246,7 @@ export async function clonePPTemplate(templateId) {
     } catch (error) {
         if (error.isAuth) return;
         console.error('Failed to clone PP template:', error);
-        showToast('Failed to clone postprocessing template', 'error');
+        showToast(t('stream.error.clone_pp_failed'), 'error');
     }
 }
 
diff --git a/server/src/wled_controller/static/js/features/targets.js b/server/src/wled_controller/static/js/features/targets.js
index 5634e01..7ca178f 100644
--- a/server/src/wled_controller/static/js/features/targets.js
+++ b/server/src/wled_controller/static/js/features/targets.js
@@ -113,9 +113,15 @@ function _updateTargetFpsChart(targetId, fpsTarget) {
     if (!chart) return;
     const actualH = _targetFpsHistory[targetId] || [];
     const currentH = _targetFpsCurrentHistory[targetId] || [];
-    chart.data.labels = actualH.map(() => '');
-    chart.data.datasets[0].data = [...actualH];
-    chart.data.datasets[1].data = [...currentH];
+    // Mutate in-place to avoid array copies
+    const ds0 = chart.data.datasets[0].data;
+    ds0.length = 0;
+    ds0.push(...actualH);
+    const ds1 = chart.data.datasets[1].data;
+    ds1.length = 0;
+    ds1.push(...currentH);
+    while (chart.data.labels.length < ds0.length) chart.data.labels.push('');
+    chart.data.labels.length = ds0.length;
     chart.options.scales.y.max = fpsTarget * 1.15;
     chart.update('none');
 }
@@ -352,7 +358,7 @@ export async function showTargetEditor(targetId = null, cloneData = null) {
         setTimeout(() => document.getElementById('target-editor-name').focus(), 100);
     } catch (error) {
         console.error('Failed to open target editor:', error);
-        showToast('Failed to open target editor', 'error');
+        showToast(t('target.error.editor_open_failed'), 'error');
     }
 }
 
@@ -972,7 +978,7 @@ export async function startTargetProcessing(targetId) {
             showToast(t('device.started'), 'success');
         } else {
             const error = await response.json();
-            showToast(`Failed to start: ${error.detail}`, 'error');
+            showToast(t('target.error.start_failed'), 'error');
         }
     });
 }
@@ -986,7 +992,7 @@ export async function stopTargetProcessing(targetId) {
             showToast(t('device.stopped'), 'success');
         } else {
             const error = await response.json();
-            showToast(`Failed to stop: ${error.detail}`, 'error');
+            showToast(t('target.error.stop_failed'), 'error');
         }
     });
 }
@@ -1027,7 +1033,7 @@ export async function cloneTarget(targetId) {
         showTargetEditor(null, target);
     } catch (error) {
         console.error('Failed to clone target:', error);
-        showToast('Failed to clone target', 'error');
+        showToast(t('target.error.clone_failed'), 'error');
     }
 }
 
@@ -1042,11 +1048,11 @@ export async function toggleTargetAutoStart(targetId, enable) {
             loadTargetsTab();
         } else {
             const error = await response.json();
-            showToast(`Failed: ${error.detail}`, 'error');
+            showToast(t('target.error.autostart_toggle_failed'), 'error');
         }
     } catch (error) {
         console.error('Failed to toggle auto-start:', error);
-        showToast('Failed to toggle auto-start', 'error');
+        showToast(t('target.error.autostart_toggle_failed'), 'error');
     }
 }
 
@@ -1062,7 +1068,7 @@ export async function deleteTarget(targetId) {
             showToast(t('targets.deleted'), 'success');
         } else {
             const error = await response.json();
-            showToast(`Failed to delete: ${error.detail}`, 'error');
+            showToast(t('target.error.delete_failed'), 'error');
         }
     });
 }
diff --git a/server/src/wled_controller/static/js/features/value-sources.js b/server/src/wled_controller/static/js/features/value-sources.js
index f829336..c318fcd 100644
--- a/server/src/wled_controller/static/js/features/value-sources.js
+++ b/server/src/wled_controller/static/js/features/value-sources.js
@@ -239,10 +239,11 @@ export async function saveValueSource() {
 export async function editValueSource(sourceId) {
     try {
         const resp = await fetchWithAuth(`/value-sources/${sourceId}`);
-        if (!resp.ok) throw new Error('fetch failed');
+        if (!resp.ok) throw new Error(t('value_source.error.load'));
         const data = await resp.json();
         await showValueSourceModal(data);
     } catch (e) {
+        if (e.isAuth) return;
         showToast(e.message, 'error');
     }
 }
@@ -252,7 +253,7 @@ export async function editValueSource(sourceId) {
 export async function cloneValueSource(sourceId) {
     try {
         const resp = await fetchWithAuth(`/value-sources/${sourceId}`);
-        if (!resp.ok) throw new Error('fetch failed');
+        if (!resp.ok) throw new Error(t('value_source.error.load'));
         const data = await resp.json();
         delete data.id;
         data.name = data.name + ' (copy)';
diff --git a/server/src/wled_controller/static/locales/en.json b/server/src/wled_controller/static/locales/en.json
index cfd40b8..55cbfdd 100644
--- a/server/src/wled_controller/static/locales/en.json
+++ b/server/src/wled_controller/static/locales/en.json
@@ -992,5 +992,54 @@
   "settings.saved_backups.download": "Download",
   "settings.saved_backups.delete": "Delete",
   "settings.saved_backups.delete_confirm": "Delete this backup file?",
-  "settings.saved_backups.delete_error": "Failed to delete backup"
+  "settings.saved_backups.delete_error": "Failed to delete backup",
+  "device.error.power_off_failed": "Failed to turn off device",
+  "device.removed": "Device removed",
+  "device.error.remove_failed": "Failed to remove device",
+  "device.error.settings_load_failed": "Failed to load device settings",
+  "device.error.brightness": "Failed to update brightness",
+  "device.error.required": "Please fill in all fields correctly",
+  "device.error.update": "Failed to update device",
+  "device.error.save": "Failed to save settings",
+  "device_discovery.error.fill_all_fields": "Please fill in all fields",
+  "device_discovery.added": "Device added successfully",
+  "device_discovery.error.add_failed": "Failed to add device",
+  "calibration.error.load_failed": "Failed to load calibration",
+  "calibration.error.css_load_failed": "Failed to load color strip source",
+  "calibration.error.test_toggle_failed": "Failed to toggle test edge",
+  "calibration.saved": "Calibration saved",
+  "calibration.error.save_failed": "Failed to save calibration",
+  "calibration.error.led_count_mismatch": "Total LEDs must equal the device LED count",
+  "calibration.error.led_count_exceeded": "Calibrated LEDs exceed the total LED count",
+  "dashboard.error.profile_toggle_failed": "Failed to toggle profile",
+  "dashboard.error.start_failed": "Failed to start processing",
+  "dashboard.error.stop_failed": "Failed to stop processing",
+  "dashboard.error.autostart_toggle_failed": "Failed to toggle auto-start",
+  "dashboard.error.stop_all": "Failed to stop all targets",
+  "target.error.editor_open_failed": "Failed to open target editor",
+  "target.error.start_failed": "Failed to start target",
+  "target.error.stop_failed": "Failed to stop target",
+  "target.error.clone_failed": "Failed to clone target",
+  "target.error.autostart_toggle_failed": "Failed to toggle auto-start",
+  "target.error.delete_failed": "Failed to delete target",
+  "audio_source.error.load": "Failed to load audio source",
+  "audio_template.error.clone_failed": "Failed to clone audio template",
+  "value_source.error.load": "Failed to load value source",
+  "color_strip.error.editor_open_failed": "Failed to open color strip editor",
+  "color_strip.error.clone_failed": "Failed to clone color strip source",
+  "color_strip.error.delete_failed": "Failed to delete color strip source",
+  "pattern.error.editor_open_failed": "Failed to open pattern template editor",
+  "pattern.error.clone_failed": "Failed to clone pattern template",
+  "pattern.error.delete_failed": "Failed to delete pattern template",
+  "pattern.error.capture_bg_failed": "Failed to capture background",
+  "stream.error.clone_picture_failed": "Failed to clone picture source",
+  "stream.error.clone_capture_failed": "Failed to clone capture template",
+  "stream.error.clone_pp_failed": "Failed to clone postprocessing template",
+  "kc_target.error.editor_open_failed": "Failed to open key colors editor",
+  "kc_target.error.clone_failed": "Failed to clone key colors target",
+  "kc_target.error.delete_failed": "Failed to delete key colors target",
+  "theme.switched.dark": "Switched to dark theme",
+  "theme.switched.light": "Switched to light theme",
+  "accent.color.updated": "Accent color updated",
+  "search.footer": "↑↓ navigate · Enter select · Esc close"
 }
diff --git a/server/src/wled_controller/static/locales/ru.json b/server/src/wled_controller/static/locales/ru.json
index 5990744..d0b292e 100644
--- a/server/src/wled_controller/static/locales/ru.json
+++ b/server/src/wled_controller/static/locales/ru.json
@@ -992,5 +992,54 @@
   "settings.saved_backups.download": "Скачать",
   "settings.saved_backups.delete": "Удалить",
   "settings.saved_backups.delete_confirm": "Удалить эту резервную копию?",
-  "settings.saved_backups.delete_error": "Не удалось удалить копию"
+  "settings.saved_backups.delete_error": "Не удалось удалить копию",
+  "device.error.power_off_failed": "Не удалось выключить устройство",
+  "device.removed": "Устройство удалено",
+  "device.error.remove_failed": "Не удалось удалить устройство",
+  "device.error.settings_load_failed": "Не удалось загрузить настройки устройства",
+  "device.error.brightness": "Не удалось обновить яркость",
+  "device.error.required": "Пожалуйста, заполните все поля",
+  "device.error.update": "Не удалось обновить устройство",
+  "device.error.save": "Не удалось сохранить настройки",
+  "device_discovery.error.fill_all_fields": "Пожалуйста, заполните все поля",
+  "device_discovery.added": "Устройство успешно добавлено",
+  "device_discovery.error.add_failed": "Не удалось добавить устройство",
+  "calibration.error.load_failed": "Не удалось загрузить калибровку",
+  "calibration.error.css_load_failed": "Не удалось загрузить источник цветовой полосы",
+  "calibration.error.test_toggle_failed": "Не удалось переключить тестовый край",
+  "calibration.saved": "Калибровка сохранена",
+  "calibration.error.save_failed": "Не удалось сохранить калибровку",
+  "calibration.error.led_count_mismatch": "Общее количество LED должно совпадать с количеством LED устройства",
+  "calibration.error.led_count_exceeded": "Калиброванных LED больше, чем общее количество LED",
+  "dashboard.error.profile_toggle_failed": "Не удалось переключить профиль",
+  "dashboard.error.start_failed": "Не удалось запустить обработку",
+  "dashboard.error.stop_failed": "Не удалось остановить обработку",
+  "dashboard.error.autostart_toggle_failed": "Не удалось переключить автозапуск",
+  "dashboard.error.stop_all": "Не удалось остановить все цели",
+  "target.error.editor_open_failed": "Не удалось открыть редактор цели",
+  "target.error.start_failed": "Не удалось запустить цель",
+  "target.error.stop_failed": "Не удалось остановить цель",
+  "target.error.clone_failed": "Не удалось клонировать цель",
+  "target.error.autostart_toggle_failed": "Не удалось переключить автозапуск",
+  "target.error.delete_failed": "Не удалось удалить цель",
+  "audio_source.error.load": "Не удалось загрузить аудиоисточник",
+  "audio_template.error.clone_failed": "Не удалось клонировать аудиошаблон",
+  "value_source.error.load": "Не удалось загрузить источник значений",
+  "color_strip.error.editor_open_failed": "Не удалось открыть редактор цветовой полосы",
+  "color_strip.error.clone_failed": "Не удалось клонировать источник цветовой полосы",
+  "color_strip.error.delete_failed": "Не удалось удалить источник цветовой полосы",
+  "pattern.error.editor_open_failed": "Не удалось открыть редактор шаблона узоров",
+  "pattern.error.clone_failed": "Не удалось клонировать шаблон узоров",
+  "pattern.error.delete_failed": "Не удалось удалить шаблон узоров",
+  "pattern.error.capture_bg_failed": "Не удалось захватить фон",
+  "stream.error.clone_picture_failed": "Не удалось клонировать источник изображения",
+  "stream.error.clone_capture_failed": "Не удалось клонировать шаблон захвата",
+  "stream.error.clone_pp_failed": "Не удалось клонировать шаблон постобработки",
+  "kc_target.error.editor_open_failed": "Не удалось открыть редактор ключевых цветов",
+  "kc_target.error.clone_failed": "Не удалось клонировать цель ключевых цветов",
+  "kc_target.error.delete_failed": "Не удалось удалить цель ключевых цветов",
+  "theme.switched.dark": "Переключено на тёмную тему",
+  "theme.switched.light": "Переключено на светлую тему",
+  "accent.color.updated": "Цвет акцента обновлён",
+  "search.footer": "↑↓ навигация · Enter выбор · Esc закрыть"
 }
diff --git a/server/src/wled_controller/static/locales/zh.json b/server/src/wled_controller/static/locales/zh.json
index 5bd9fb4..e00f75a 100644
--- a/server/src/wled_controller/static/locales/zh.json
+++ b/server/src/wled_controller/static/locales/zh.json
@@ -992,5 +992,54 @@
   "settings.saved_backups.download": "下载",
   "settings.saved_backups.delete": "删除",
   "settings.saved_backups.delete_confirm": "删除此备份文件？",
-  "settings.saved_backups.delete_error": "删除备份失败"
+  "settings.saved_backups.delete_error": "删除备份失败",
+  "device.error.power_off_failed": "关闭设备失败",
+  "device.removed": "设备已移除",
+  "device.error.remove_failed": "移除设备失败",
+  "device.error.settings_load_failed": "加载设备设置失败",
+  "device.error.brightness": "更新亮度失败",
+  "device.error.required": "请填写所有字段",
+  "device.error.update": "更新设备失败",
+  "device.error.save": "保存设置失败",
+  "device_discovery.error.fill_all_fields": "请填写所有字段",
+  "device_discovery.added": "设备添加成功",
+  "device_discovery.error.add_failed": "添加设备失败",
+  "calibration.error.load_failed": "加载校准失败",
+  "calibration.error.css_load_failed": "加载色带源失败",
+  "calibration.error.test_toggle_failed": "切换测试边缘失败",
+  "calibration.saved": "校准已保存",
+  "calibration.error.save_failed": "保存校准失败",
+  "calibration.error.led_count_mismatch": "LED总数必须等于设备LED数量",
+  "calibration.error.led_count_exceeded": "校准的LED超过了LED总数",
+  "dashboard.error.profile_toggle_failed": "切换配置文件失败",
+  "dashboard.error.start_failed": "启动处理失败",
+  "dashboard.error.stop_failed": "停止处理失败",
+  "dashboard.error.autostart_toggle_failed": "切换自动启动失败",
+  "dashboard.error.stop_all": "停止所有目标失败",
+  "target.error.editor_open_failed": "打开目标编辑器失败",
+  "target.error.start_failed": "启动目标失败",
+  "target.error.stop_failed": "停止目标失败",
+  "target.error.clone_failed": "克隆目标失败",
+  "target.error.autostart_toggle_failed": "切换自动启动失败",
+  "target.error.delete_failed": "删除目标失败",
+  "audio_source.error.load": "加载音频源失败",
+  "audio_template.error.clone_failed": "克隆音频模板失败",
+  "value_source.error.load": "加载数值源失败",
+  "color_strip.error.editor_open_failed": "打开色带编辑器失败",
+  "color_strip.error.clone_failed": "克隆色带源失败",
+  "color_strip.error.delete_failed": "删除色带源失败",
+  "pattern.error.editor_open_failed": "打开图案模板编辑器失败",
+  "pattern.error.clone_failed": "克隆图案模板失败",
+  "pattern.error.delete_failed": "删除图案模板失败",
+  "pattern.error.capture_bg_failed": "捕获背景失败",
+  "stream.error.clone_picture_failed": "克隆图片源失败",
+  "stream.error.clone_capture_failed": "克隆捕获模板失败",
+  "stream.error.clone_pp_failed": "克隆后处理模板失败",
+  "kc_target.error.editor_open_failed": "打开关键颜色编辑器失败",
+  "kc_target.error.clone_failed": "克隆关键颜色目标失败",
+  "kc_target.error.delete_failed": "删除关键颜色目标失败",
+  "theme.switched.dark": "已切换到深色主题",
+  "theme.switched.light": "已切换到浅色主题",
+  "accent.color.updated": "强调色已更新",
+  "search.footer": "↑↓ 导航 · Enter 选择 · Esc 关闭"
 }
diff --git a/server/src/wled_controller/templates/index.html b/server/src/wled_controller/templates/index.html
index c1aba40..9d84b7c 100644
--- a/server/src/wled_controller/templates/index.html
+++ b/server/src/wled_controller/templates/index.html
@@ -170,7 +170,7 @@
         <div class="cp-dialog">
             <input id="cp-input" class="cp-input" placeholder="Search..." autocomplete="off">
             <div id="cp-results" class="cp-results"></div>
-            <div class="cp-footer">↑↓ navigate · Enter select · Esc close</div>
+            <div class="cp-footer" data-i18n="search.footer">↑↓ navigate · Enter select · Esc close</div>
         </div>
     </div>
 
@@ -198,7 +198,7 @@
             // Re-derive accent text variant for the new theme
             const accent = localStorage.getItem('accentColor');
             if (accent) applyAccentColor(accent, true);
-            showToast(`Switched to ${newTheme} theme`, 'info');
+            showToast(window.t ? t(newTheme === 'dark' ? 'theme.switched.dark' : 'theme.switched.light') : `Switched to ${newTheme} theme`, 'info');
         }
 
         // Initialize accent color
@@ -247,7 +247,7 @@
             if (native) native.value = hex;
             localStorage.setItem('accentColor', hex);
             document.dispatchEvent(new CustomEvent('accentColorChanged', { detail: { color: hex } }));
-            if (!silent) showToast('Accent color updated', 'info');
+            if (!silent) showToast(window.t ? t('accent.color.updated') : 'Accent color updated', 'info');
         }
 
         // Bootstrap _cpToggle/_cpPick globals before the color-picker module loads
diff --git a/server/src/wled_controller/templates/modals/audio-source-editor.html b/server/src/wled_controller/templates/modals/audio-source-editor.html
index 2e83944..c897aaf 100644
--- a/server/src/wled_controller/templates/modals/audio-source-editor.html
+++ b/server/src/wled_controller/templates/modals/audio-source-editor.html
@@ -1,9 +1,9 @@
 <!-- Audio Source Editor Modal -->
-<div id="audio-source-modal" class="modal" role="dialog" aria-labelledby="audio-source-modal-title">
+<div id="audio-source-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="audio-source-modal-title">
     <div class="modal-content">
         <div class="modal-header">
             <h2 id="audio-source-modal-title" data-i18n="audio_source.add">Add Audio Source</h2>
-            <button class="modal-close-btn" onclick="closeAudioSourceModal()" aria-label="Close">&times;</button>
+            <button class="modal-close-btn" onclick="closeAudioSourceModal()" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">
             <form id="audio-source-form" onsubmit="return false;">
diff --git a/server/src/wled_controller/templates/modals/confirm.html b/server/src/wled_controller/templates/modals/confirm.html
index 8e2154a..acdb77b 100644
--- a/server/src/wled_controller/templates/modals/confirm.html
+++ b/server/src/wled_controller/templates/modals/confirm.html
@@ -9,8 +9,8 @@
             <p id="confirm-message" class="modal-description"></p>
         </div>
         <div class="modal-footer">
-            <button class="btn btn-danger" id="confirm-no-btn" onclick="closeConfirmModal(false)">No</button>
-            <button class="btn btn-secondary" id="confirm-yes-btn" onclick="closeConfirmModal(true)">Yes</button>
+            <button class="btn btn-secondary" id="confirm-no-btn" onclick="closeConfirmModal(false)">No</button>
+            <button class="btn btn-danger" id="confirm-yes-btn" onclick="closeConfirmModal(true)">Yes</button>
         </div>
     </div>
 </div>
diff --git a/server/src/wled_controller/templates/modals/settings.html b/server/src/wled_controller/templates/modals/settings.html
index ea19752..a9b0ef5 100644
--- a/server/src/wled_controller/templates/modals/settings.html
+++ b/server/src/wled_controller/templates/modals/settings.html
@@ -75,5 +75,8 @@
 
             <div id="settings-error" class="error-message" style="display:none;"></div>
         </div>
+        <div class="modal-footer">
+            <button class="btn btn-icon btn-secondary" onclick="closeSettingsModal()" title="Close" data-i18n-title="settings.button.cancel" data-i18n-aria-label="aria.close">&#x2715;</button>
+        </div>
     </div>
 </div>
diff --git a/server/src/wled_controller/templates/modals/value-source-editor.html b/server/src/wled_controller/templates/modals/value-source-editor.html
index 2fd545f..4229d5f 100644
--- a/server/src/wled_controller/templates/modals/value-source-editor.html
+++ b/server/src/wled_controller/templates/modals/value-source-editor.html
@@ -1,9 +1,9 @@
 <!-- Value Source Editor Modal -->
-<div id="value-source-modal" class="modal" role="dialog" aria-labelledby="value-source-modal-title">
+<div id="value-source-modal" class="modal" role="dialog" aria-modal="true" aria-labelledby="value-source-modal-title">
     <div class="modal-content">
         <div class="modal-header">
             <h2 id="value-source-modal-title" data-i18n="value_source.add">Add Value Source</h2>
-            <button class="modal-close-btn" onclick="closeValueSourceModal()" aria-label="Close">&times;</button>
+            <button class="modal-close-btn" onclick="closeValueSourceModal()" data-i18n-aria-label="aria.close">&#x2715;</button>
         </div>
         <div class="modal-body">
             <form id="value-source-form" onsubmit="return false;">