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/js/main.js

@@ -25,6 +25,9 @@ window.addEventListener('DOMContentLoaded', async () => {
         }
     });
 
+    // Initialize dynamic background
+    applyDynamicBackground();
+
     // Initialize locale (async - loads JSON file)
     await initLocale();