Add dynamic WebGL background with audio reactivity

- WebGL shader background with flowing waves, radial pulse, and frequency ring arcs
- Reacts to captured audio data (frequency bands + bass) when visualizer is active
- Uses page accent color; adapts to dark/light theme via bg-primary blending
- Toggle button in header toolbar, state persisted in localStorage
- Cached uniform locations and color values to avoid per-frame getComputedStyle calls
- i18n support for EN/RU locales

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

media_server/static/locales/en.json

@@ -24,6 +24,7 @@
   "player.unknown_source": "Unknown",
   "player.vinyl": "Vinyl mode",
   "player.visualizer": "Audio visualizer",
+  "player.background": "Dynamic background",
   "state.playing": "Playing",
   "state.paused": "Paused",
   "state.stopped": "Stopped",