fix: cover art on auto-advance + native back-button overlay routing

Issue 1 (MusicService): wrap DefaultHttpDataSource in CacheBitmapLoader so
Media3 notification artwork is fetched once and served from memory on every
subsequent auto-advance. warmArtworkCache() pre-loads current+next 3 tracks
on both onMediaItemTransition and onTimelineChanged (queue replace via setQueue).
Adds explicit media3-datasource dependency.

Issue 2 (back button): replace deprecated onKeyDown with OnBackPressedCallback.
WebBridge.setOverlayState(bool) tracks whether JS has an overlay open;
consumeBackPress() is called first — if true, MainActivity delegates to
window.NSPlayer.handleBack() which closes the topmost layer. Falls back to
webView.goBack() then moveTaskToBack(true) so the MusicService binding survives.
JS side: _syncOverlayState() + MutationObserver keep the native flag current;
handleBack covers NP overlay, queue panel, action sheet, all modals, sidebar.

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

src/player.js

@@ -710,6 +710,7 @@ export function toggleQueue() {
   S.queueOpen = !S.queueOpen;
   document.getElementById('queue-panel').classList.toggle('open', S.queueOpen);
   if(S.queueOpen) renderQueue();
+  _syncOverlayState();
 }
 
 // ─── DOWNLOAD ─────────────────────────────────────────────────────────────
@@ -767,6 +768,83 @@ export function updatePreviewProgress() {
 
 // ─── RESTORE LAST PLAYED ──────────────────────────────────────────────────
 
+// ─── BACK-BUTTON / OVERLAY STATE SYNC ────────────────────────────────────
+// Native back button: when JS has an overlay open, Android delegates the event
+// to window.NSPlayer.handleBack() instead of minimising the app.
+
+function _hasAnyOverlayOpen() {
+  if (S.npOpen || S.queueOpen) return true;
+  if (document.getElementById('np-sheet')?.classList.contains('open')) return true;
+  if (document.getElementById('ctx-menu')?.style.display === 'block') return true;
+  if (document.getElementById('sidebar')?.classList.contains('mobile-open')) return true;
+  return ['settings-modal','pl-modal','info-modal','dl-modal','edit-pl-modal','confirm-modal','notif-panel']
+    .some(id => document.getElementById(id)?.classList.contains('open'));
+}
+
+export function _syncOverlayState() {
+  if (!HAS_BRIDGE) return;
+  try { window.NSBridge.setOverlayState(_hasAnyOverlayOpen()); } catch {}
+}
+
+window.NSPlayer.handleBack = function() {
+  // Close overlays from topmost layer downward.
+  if (document.getElementById('np-sheet')?.classList.contains('open')) {
+    window.closeNpMenu?.(); _syncOverlayState(); return;
+  }
+  const ctxMenu = document.getElementById('ctx-menu');
+  if (ctxMenu?.style.display === 'block') {
+    ctxMenu.style.display = 'none'; ctxMenu.innerHTML = '';
+    _syncOverlayState(); return;
+  }
+  if (S.npOpen) {
+    window.closeNowPlaying?.(); _syncOverlayState(); return;
+  }
+  if (S.queueOpen) {
+    toggleQueue(); _syncOverlayState(); return;
+  }
+  // Generic modals
+  const modalMap = {
+    'settings-modal': 'closeSettings',
+    'pl-modal':       'closePlModal',
+    'info-modal':     'closeInfoModal',
+    'dl-modal':       'closeDlModal',
+    'edit-pl-modal':  'closeEditPlModal',
+    'confirm-modal':  '_confirmCancel',
+  };
+  for (const [id, fn] of Object.entries(modalMap)) {
+    const el = document.getElementById(id);
+    if (el?.classList.contains('open')) {
+      if (window[fn]) window[fn](); else el.classList.remove('open');
+      _syncOverlayState(); return;
+    }
+  }
+  // Notif panel (generic fallback)
+  const notifPanel = document.getElementById('notif-panel');
+  if (notifPanel?.classList.contains('open')) {
+    notifPanel.classList.remove('open'); _syncOverlayState(); return;
+  }
+  // Sidebar
+  if (document.getElementById('sidebar')?.classList.contains('mobile-open')) {
+    window.closeSidebar?.(); _syncOverlayState(); return;
+  }
+};
+
+// MutationObserver keeps hasOpenOverlay in sync whenever the DOM changes class
+// attributes (e.g. any overlay gains/loses the 'open' class).
+if (HAS_BRIDGE) {
+  const _startOverlayObserver = () => {
+    if (!document.body) return;
+    new MutationObserver(_syncOverlayState).observe(document.body, {
+      attributes: true, attributeFilter: ['class'], subtree: true,
+    });
+  };
+  if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', _startOverlayObserver);
+  } else {
+    _startOverlayObserver();
+  }
+}
+
 export function _restoreLastPlayed() {
   try {
     const saved = JSON.parse(localStorage.getItem('ns_last_played') || 'null');