feat: Add Media Session API support for browser media controls

- Implement `updateMediaSession()` to sync track metadata (title, artist, artwork) with browser's media session
- Update playback state (`playing`/`paused`) via `mediaSession.playbackState`
- Register action handlers for common media controls: play, pause, next/prev track, seek operations
- Integrate `updateMediaSession()` call into `playCurrent()` to show metadata when playback starts

Enables native browser media notification and lock screen controls on supported platforms.

public/js/player.js

@@ -62,6 +62,19 @@
     });
   }
 
+  function updateMediaSession() {
+    if (!('mediaSession' in navigator)) return;
+    const cur = state.queue[state.index];
+    if (!cur) return;
+    navigator.mediaSession.metadata = new MediaMetadata({
+      title:  cur.title,
+      artist: cur.artist || '',
+      artwork: cur.cover
+        ? [{ src: new URL(cur.cover, window.location.origin).href, sizes: '512x512', type: 'image/jpeg' }]
+        : [],
+    });
+  }
+
   function playCurrent() {
     const cur = state.queue[state.index];
     if (!cur) return;
@@ -72,6 +85,7 @@
       .catch((err) => console.warn('play failed', err));
     render();
     highlightNowPlaying(cur.id);
+    updateMediaSession();
   }
 
   function playOne(song) {
@@ -98,9 +112,25 @@
     }
   }
 
+  function prev() {
+    if (state.index > 0) { state.index -= 1; playCurrent(); }
+  }
+
   audio.addEventListener('ended', next);
+  audio.addEventListener('play',  () => { if ('mediaSession' in navigator) navigator.mediaSession.playbackState = 'playing'; });
+  audio.addEventListener('pause', () => { if ('mediaSession' in navigator) navigator.mediaSession.playbackState = 'paused'; });
   nextBtn.addEventListener('click', next);
 
+  if ('mediaSession' in navigator) {
+    navigator.mediaSession.setActionHandler('play',          () => audio.play());
+    navigator.mediaSession.setActionHandler('pause',         () => audio.pause());
+    navigator.mediaSession.setActionHandler('nexttrack',     () => next());
+    navigator.mediaSession.setActionHandler('previoustrack', () => prev());
+    navigator.mediaSession.setActionHandler('seekto',        (d) => { audio.currentTime = d.seekTime; });
+    navigator.mediaSession.setActionHandler('seekbackward',  (d) => { audio.currentTime = Math.max(0, audio.currentTime - (d.seekOffset || 10)); });
+    navigator.mediaSession.setActionHandler('seekforward',   (d) => { audio.currentTime = Math.min(audio.duration || Infinity, audio.currentTime + (d.seekOffset || 10)); });
+  }
+
   repeatBtn.addEventListener('click', () => {
     repeatOn = !repeatOn;
     repeatBtn.classList.toggle('active', repeatOn);