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

server/src/wled_controller/templates/index.html

@@ -212,6 +212,8 @@
     {% include 'modals/test-value-source.html' %}
     {% include 'modals/sync-clock-editor.html' %}
     {% include 'modals/weather-source-editor.html' %}
+    {% include 'modals/asset-upload.html' %}
+    {% include 'modals/asset-editor.html' %}
     {% include 'modals/settings.html' %}
 
     {% include 'partials/tutorial-overlay.html' %}