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/utils/file_ops.py

@@ -1,10 +1,13 @@
 """Atomic file write utilities."""
 
 import json
+import logging
 import os
 import tempfile
 from pathlib import Path
 
+logger = logging.getLogger(__name__)
+
 
 def atomic_write_json(file_path: Path, data: dict, indent: int = 2) -> None:
     """Write JSON data to file atomically via temp file + rename.
@@ -29,6 +32,7 @@ def atomic_write_json(file_path: Path, data: dict, indent: int = 2) -> None:
         # Clean up temp file on any error
         try:
             os.unlink(tmp_path)
-        except OSError:
+        except OSError as e:
+            logger.debug("Failed to clean up temp file %s: %s", tmp_path, e)
             pass
         raise