Replace plain <input type="file"> with a styled drag-and-drop zone
featuring hover/drag states, file info display, and remove button.
Fix 10 review issues: add 401 handling to upload, remove non-null
assertions, add missing i18n keys (common.remove, error.play_failed,
error.download_failed), normalize close button glyphs to ✕,
i18n the dropzone aria-label, replace silent error catches with toast
notifications, use DataTransfer for cross-browser file assignment.
- 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
