fix(mobile): restore playback state + MediaSession stability

- _restoreLastPlayed: set audio.src + call updateMediaSession so play
  button works immediately and notification shows artwork on app open
- visibilitychange: re-register all MediaSession handlers after screen
  unlock, fold/flip, or background return (fixes vanishing notification)
- Stall watchdog (3s interval): detects frozen audio in background and
  calls play() to recover; also catches missed 'ended' events so
  auto-advance works reliably when screen is off
- stalled/error listeners: attempt audio reload on network hiccup

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

src/player.js

@@ -353,5 +353,9 @@ export function _restoreLastPlayed() {
     art.innerHTML = cv ? `<img src="${cv}" style="width:100%;height:100%;object-fit:cover">` : '<div class="no-art">♪</div>';
     updateStarBtn();
     document.title = `${s.title} — NovaSonic`;
+    // set src so the play button works immediately without needing to re-select the song
+    S.audio.src = streamUrl(s.id);
+    S.audio.load();
+    updateMediaSession(s);
   } catch {}
 }