PlayerBar reads playbackState + mediaItem streams; hidden when no
queue. Home track-row tap, album play button, album track tap, artist
play button (shuffle) all route to playerActions.playTracks. Token +
baseUrl forwarded to the audio handler before each new queue.
audio_service runs the handler in a background isolate; UI watches
playbackState/mediaItem/queue streams through Riverpod. AndroidManifest
gets the foreground-service media-playback permission; Info.plist gets
UIBackgroundModes=audio. Bearer token attached to stream URLs via
just_audio's per-source headers.
bvandeusen