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/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