fix: stable notification artwork + canplay retry for background audio

- Cache cover art URLs per song so the same auth token is reused across
  metadata updates; Samsung/Chrome can cache the artwork image and the
  notification no longer visually reloads on every skip
- Add canplay listener as background-safe play() retry: the audio
  element fires canplay from its internal state machine (not throttled
  like setTimeout by Android Doze), so play() is retried as soon as
  the browser has buffered enough data
- Set navigator.audioSession.type='playback' on each _tryPlay attempt
  (not just on play event) so the OS audio session claim happens before
  Chrome requests the play grant
- Guard _onForeground playbackState assignment with !_trackChanging to
  prevent visibilitychange/pageshow from stomping 'playing' to 'paused'
  during an in-progress track transition

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.js

@@ -166,7 +166,8 @@ window.addEventListener('load', () => {
     const s = S.queue[S.qIdx];
     if(!s) return;
     updateMediaSession(s);
-    if(navigator.mediaSession)
+    // Don't stomp playbackState during a src-swap — _trackChanging guards the transition
+    if(navigator.mediaSession && !S._trackChanging)
       navigator.mediaSession.playbackState = S.audio.paused ? 'paused' : 'playing';
     // If audio was playing before backgrounding but is now paused (OS kill), try to resume
     if(!S.audio.paused) return;