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/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';
     }
 }