When selecting the builtin/local player from the player selector overlay,
the track info would disappear because the queue API returns null for
local players. The code was clearing _currentTrack unconditionally.
Now for the builtin player:
- Preserve cached track data instead of clearing it
- Restore from cache if _currentTrack is null
- Update notification with actual track info when available
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
