Users need to reorder folders and bookmarks via drag-and-drop (including mobile/touch) and have that order persist across reloads. Current HTML5 drag/drop is unreliable on mobile and ordering is not stored for bookmarks.
## What Changes
- Add persistent ordering for bookmarks (new DB column and API endpoint to reorder within a folder).
- Use a touch-friendly drag-and-drop implementation in the web UI for:
- Reordering folders within the same parent.
- Reordering bookmarks within the same folder.
- Keep the root group (no folder) as a first-class group in the UI.