Playlist & Favorites:
- Fix playlist loading error (missing field `id` in JSON parsing)
- Add serde defaults for Track and PlaylistOwner optional fields
- Fix track favorites toggle to actually call backend API
- Add "Add to playlist" button on track rows in album view
Sidebar UI improvements:
- Increase sidebar width to 280px
- Reduce text size and margins throughout
- Normalize all icons to 14px (matching + and refresh buttons)
- Add text overflow with ellipsis and tooltip for playlist names
User card:
- Make user card more compact (smaller avatar, fonts, icons)
- Add collapsible toggle - when collapsed shows mini QBZ logo + username
Queue & Playback:
- Fix next_track spam when queue has only one song (queueEnded flag)
Other fixes:
- Fix TrackRow prop types (remove invalid hires, bitDepth, samplingRate props)
- Fix FavoritesView and PlaylistDetailView component prop types
- Add diagnostic logging to credential persistence