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/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)';