feat(queue): add scroll to currently playing button

Adds a scrollToIndex method to VirtualList and a locate button in the
Queue header that scrolls to center the currently playing track.

src/lib/components/ui/VirtualList.svelte

@@ -56,6 +56,14 @@
     scrollTop = (e.target as HTMLDivElement).scrollTop;
   }
 
+  export function scrollToIndex(index: number) {
+    if (!containerEl) return;
+    containerEl.scrollTop = Math.max(
+      0,
+      index * itemHeight - containerHeight / 2 + itemHeight / 2,
+    );
+  }
+
   $effect(() => {
     if (!containerEl) return;
     const ro = new ResizeObserver((entries) => {