Show YouTube-sourced playlists with a YouTube icon; they're read-only for now (no rename/delete/reorder/remove, excluded from the add-to-playlist popover) with a 'synced from YouTube' note — editing comes with the write-back phase. Add a 'Sync from YouTube' button in the Playlists rail (api.syncYoutubePlaylists) with a result toast. Trilingual strings.