fix: prevent notification close on skip and background audio stall

- playCurrent: set _trackChanging flag and update media session metadata
  before changing src, so the pause event fired by src-swap does not
  set playbackState='paused' and dismiss the Samsung cover screen notification
- Remove stalled->load() handler that silently paused audio with no recovery
- Error handler now retries regardless of paused state (load() causes paused)
- Add .catch() to audio.play() in playCurrent

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

src/state.js

@@ -13,6 +13,7 @@ export const S = {
   previewCurrent: null,
   _scrubbing: false,
   _msPosLast: 0,
+  _trackChanging: false,
   queueOpen: false,
 
   // Now Playing overlay