Add callback management API/UI and theme support

- Add callback CRUD endpoints (create, update, delete, list)
- Add callback management UI with all 11 callback events support
- Add light/dark theme switcher with localStorage persistence
- Improve button styling (wider buttons, simplified text)
- Extend ConfigManager with callback operations

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

media_server/routes/__init__.py

@@ -1,8 +1,9 @@
 """API route modules."""
 
 from .audio import router as audio_router
+from .callbacks import router as callbacks_router
 from .health import router as health_router
 from .media import router as media_router
 from .scripts import router as scripts_router
 
-__all__ = ["audio_router", "health_router", "media_router", "scripts_router"]
+__all__ = ["audio_router", "callbacks_router", "health_router", "media_router", "scripts_router"]