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>

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