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/static/css/automations.css

@@ -215,21 +215,6 @@
     align-items: center;
 }
 
-.btn-browse-apps {
-    background: none;
-    border: 1px solid var(--border-color);
-    color: var(--text-color);
-    font-size: 0.75rem;
-    padding: 2px 8px;
-    border-radius: 4px;
-    cursor: pointer;
-    transition: border-color 0.2s, background 0.2s;
-}
-
-.btn-browse-apps:hover {
-    border-color: var(--primary-color);
-    background: rgba(33, 150, 243, 0.1);
-}
 
 /* Webhook URL row */
 .webhook-url-row {