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/locales/en.json

@@ -992,5 +992,54 @@
   "settings.saved_backups.download": "Download",
   "settings.saved_backups.delete": "Delete",
   "settings.saved_backups.delete_confirm": "Delete this backup file?",
-  "settings.saved_backups.delete_error": "Failed to delete backup"
+  "settings.saved_backups.delete_error": "Failed to delete backup",
+  "device.error.power_off_failed": "Failed to turn off device",
+  "device.removed": "Device removed",
+  "device.error.remove_failed": "Failed to remove device",
+  "device.error.settings_load_failed": "Failed to load device settings",
+  "device.error.brightness": "Failed to update brightness",
+  "device.error.required": "Please fill in all fields correctly",
+  "device.error.update": "Failed to update device",
+  "device.error.save": "Failed to save settings",
+  "device_discovery.error.fill_all_fields": "Please fill in all fields",
+  "device_discovery.added": "Device added successfully",
+  "device_discovery.error.add_failed": "Failed to add device",
+  "calibration.error.load_failed": "Failed to load calibration",
+  "calibration.error.css_load_failed": "Failed to load color strip source",
+  "calibration.error.test_toggle_failed": "Failed to toggle test edge",
+  "calibration.saved": "Calibration saved",
+  "calibration.error.save_failed": "Failed to save calibration",
+  "calibration.error.led_count_mismatch": "Total LEDs must equal the device LED count",
+  "calibration.error.led_count_exceeded": "Calibrated LEDs exceed the total LED count",
+  "dashboard.error.profile_toggle_failed": "Failed to toggle profile",
+  "dashboard.error.start_failed": "Failed to start processing",
+  "dashboard.error.stop_failed": "Failed to stop processing",
+  "dashboard.error.autostart_toggle_failed": "Failed to toggle auto-start",
+  "dashboard.error.stop_all": "Failed to stop all targets",
+  "target.error.editor_open_failed": "Failed to open target editor",
+  "target.error.start_failed": "Failed to start target",
+  "target.error.stop_failed": "Failed to stop target",
+  "target.error.clone_failed": "Failed to clone target",
+  "target.error.autostart_toggle_failed": "Failed to toggle auto-start",
+  "target.error.delete_failed": "Failed to delete target",
+  "audio_source.error.load": "Failed to load audio source",
+  "audio_template.error.clone_failed": "Failed to clone audio template",
+  "value_source.error.load": "Failed to load value source",
+  "color_strip.error.editor_open_failed": "Failed to open color strip editor",
+  "color_strip.error.clone_failed": "Failed to clone color strip source",
+  "color_strip.error.delete_failed": "Failed to delete color strip source",
+  "pattern.error.editor_open_failed": "Failed to open pattern template editor",
+  "pattern.error.clone_failed": "Failed to clone pattern template",
+  "pattern.error.delete_failed": "Failed to delete pattern template",
+  "pattern.error.capture_bg_failed": "Failed to capture background",
+  "stream.error.clone_picture_failed": "Failed to clone picture source",
+  "stream.error.clone_capture_failed": "Failed to clone capture template",
+  "stream.error.clone_pp_failed": "Failed to clone postprocessing template",
+  "kc_target.error.editor_open_failed": "Failed to open key colors editor",
+  "kc_target.error.clone_failed": "Failed to clone key colors target",
+  "kc_target.error.delete_failed": "Failed to delete key colors target",
+  "theme.switched.dark": "Switched to dark theme",
+  "theme.switched.light": "Switched to light theme",
+  "accent.color.updated": "Accent color updated",
+  "search.footer": "↑↓ navigate · Enter select · Esc close"
 }