feat: implement preloading of next track in queue

- Add a hidden `preloadAudio` element to buffer the next song in the queue
- Automatically preload the next track after the current one starts playing
- Update service worker cache version to v6 to ensure fresh assets are fetched

public/js/player.js

@@ -123,10 +123,12 @@
     highlightNowPlaying(cur.id);
     updateMediaSession();
     showPlayNotification(cur);
+    preloadNext();
   }
 
   function playOne(song) {
     requestNotificationPermission();
+    preloadAudio.src = '';
     state.queue = [song];
     state.index = 0;
     playCurrent();
@@ -159,6 +161,19 @@
   // event fired by src reassignment doesn't mislead iOS into thinking playback stopped.
   let transitioning = false;
 
+  // Hidden audio element used to buffer the next track in the queue.
+  const preloadAudio = new Audio();
+  preloadAudio.preload = 'auto';
+
+  function preloadNext() {
+    const next = state.queue[state.index + 1];
+    if (next) {
+      preloadAudio.src = `/stream/${next.id}`;
+    } else {
+      preloadAudio.src = '';
+    }
+  }
+
   audio.addEventListener('ended', next);
   audio.addEventListener('play',  () => {
     transitioning = false;

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'bri-tunes-v5';
+const CACHE = 'bri-tunes-v6';
 const STATIC_PRECACHE = [
   '/',
   '/static/css/app.css',