Codebase review: stability, performance, usability, and i18n fixes

Stability:
- Fix race condition: set _is_running before create_task in target processors
- Await probe task after cancel in wled_target_processor
- Replace raw fetch() with fetchWithAuth() across devices, kc-targets, pattern-templates
- Add try/catch to showTestTemplateModal in streams.js
- Wrap blocking I/O in asyncio.to_thread (picture_targets, system restore)
- Fix dashboardStopAll to filter only running targets with ok guard

Performance:
- Vectorize fire effect spark loop with numpy in effect_stream
- Vectorize FFT band binning with cumulative sum in analysis.py
- Rewrite pixel_processor with vectorized numpy (accept ndarray or list)
- Add httpx.AsyncClient connection pooling with lock in wled_provider
- Optimize _send_pixels_http to avoid np.hstack allocation in wled_client
- Mutate chart arrays in-place in dashboard, perf-charts, targets
- Merge dashboard 2-batch fetch into single Promise.all
- Hoist frame_time outside loop in mapped_stream

Usability:
- Fix health check interval load/save in device settings
- Swap confirm modal button classes (No=secondary, Yes=danger)
- Add aria-modal to audio/value source editors, fix close button aria-labels
- Add modal footer close button to settings modal
- Add dedicated calibration LED count validation error keys

i18n:
- Replace ~50 hardcoded English strings with t() calls across 12 JS files
- Add 50 new keys to en.json, ru.json, zh.json
- Localize inline toasts in index.html with window.t fallback
- Add data-i18n to command palette footer
- Add localization policy to CLAUDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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()