add previous/next track button handling

src/routes/streams/[stream_id]/Player.svelte

@@ -92,6 +92,12 @@
         const volumeData = localStorage.getItem('volume');
         volume = volumeData ? parseFloat(volumeData) : 0.67;
         setVolume(volume);
+
+        // set actions for previous/next track media buttons
+        if ('mediaSession' in navigator) {
+            navigator.mediaSession.setActionHandler('previoustrack', previousTrack);
+            navigator.mediaSession.setActionHandler('nexttrack', nextTrack);
+        }
     });
 
     // update browser metadata on track changes
@@ -101,6 +107,26 @@
         }
     });
 
+    function previousTrack() {
+        if (!ctx.current || ctx.songIndex == null) return;
+        const tracks = ctx.current.tracks;
+        const trackStart = tracks[ctx.songIndex][0];
+        // if more than 3s into the track, restart it; otherwise go to previous
+        if (audio.currentTime - trackStart > 3 || ctx.songIndex === 0) {
+            audio.currentTime = currentTime = trackStart;
+        } else {
+            audio.currentTime = currentTime = tracks[ctx.songIndex - 1][0];
+        }
+    }
+
+    function nextTrack() {
+        if (!ctx.current || ctx.songIndex == null) return;
+        const tracks = ctx.current.tracks;
+        if (ctx.songIndex < tracks.length - 1) {
+            audio.currentTime = currentTime = tracks[ctx.songIndex + 1][0];
+        }
+    }
+
     function seek(event, bounds) {
         let x = event.clientX - bounds.left;
         return Math.min(Math.max(x / bounds.width, 0), 1);