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

README.md

@@ -121,6 +121,26 @@ Open **http://localhost:8080** to access the dashboard.
 
 See [INSTALLATION.md](INSTALLATION.md) for the full installation guide, including configuration, Docker manual builds, and Home Assistant setup.
 
+## Demo Mode
+
+Demo mode runs the server with virtual devices, sample data, and isolated storage — useful for exploring the UI without real hardware.
+
+Set the `WLED_DEMO` environment variable to `true`, `1`, or `yes`:
+
+```bash
+# Docker
+docker compose run -e WLED_DEMO=true server
+
+# Python
+WLED_DEMO=true uvicorn wled_controller.main:app --host 0.0.0.0 --port 8081
+
+# Windows (installed app)
+set WLED_DEMO=true
+LedGrab.bat
+```
+
+Demo mode uses port **8081**, config file `config/demo_config.yaml`, and stores data in `data/demo/` (separate from production data). It can run alongside the main server.
+
 ## Architecture
 
 ```text