feat: asset-based image/video sources, notification sounds, UI improvements

- Replace URL-based image_source/url fields with image_asset_id/video_asset_id
  on StaticImagePictureSource and VideoCaptureSource (clean break, no migration)
- Resolve asset IDs to file paths at runtime via AssetStore.get_file_path()
- Add EntitySelect asset pickers for image/video in stream editor modal
- Add notification sound configuration (global sound + per-app overrides)
- Unify per-app color and sound overrides into single "Per-App Overrides" section
- Persist notification history between server restarts
- Add asset management system (upload, edit, delete, soft-delete)
- Replace emoji buttons with SVG icons throughout UI
- Various backend improvements: SQLite stores, auth, backup, MQTT, webhooks

contexts/server-operations.md

@@ -11,7 +11,9 @@ Two independent server modes with separate configs, ports, and data directories:
 | **Real** | `python -m wled_controller` | `config/default_config.yaml` | 8080 | `development-key-change-in-production` | `data/` |
 | **Demo** | `python -m wled_controller.demo` | `config/demo_config.yaml` | 8081 | `demo` | `data/demo/` |
 
-Both can run simultaneously on different ports.
+Demo mode can also be triggered via the `WLED_DEMO` environment variable (`true`, `1`, or `yes`). This works with any launch method — Python, Docker (`-e WLED_DEMO=true`), or the installed app (`set WLED_DEMO=true` before `LedGrab.bat`).
+
+Both modes can run simultaneously on different ports.
 
 ## Restart Procedure