fix(lyrics): keep highlight + auto-scroll alive after tab switch (#454)

The track-change reset wiped lineRefs/wordRefs in its effect body, which
ran *after* the commit phase that just populated them on remount. The
tracker effect then saw an empty refs array, no-op'd the DOM update, but
still advanced prevActive — so every following progress tick early-returned
on `prev.line === lineIdx` and the highlight froze.

Track the previous track id in a ref and skip the reset on initial mount,
so refs only get cleared on an actual track change.

src/components/LyricsPane.tsx

@@ -38,6 +38,7 @@ export default function LyricsPane({ currentTrack }: Props) {
   const lineRefs      = useRef<(HTMLDivElement | null)[]>([]);
   const wordRefs      = useRef<HTMLSpanElement[][]>([]);
   const prevActive    = useRef({ line: -1, word: -1 });
+  const prevTrackId   = useRef<string | null | undefined>(undefined);
   const isUserScroll  = useRef(false);
   const scrollTimer   = useRef<ReturnType<typeof setTimeout> | null>(null);
 
@@ -65,12 +66,18 @@ export default function LyricsPane({ currentTrack }: Props) {
     }
   }, [sidebarLyricsStyle]);
 
-  // Reset refs when track changes.
+  // Reset refs on actual track change. Skip the initial mount so we don't wipe
+  // the callback-refs that the commit phase just populated — that would leave
+  // the tracker effect with an empty lineRefs and freeze the highlight.
   useEffect(() => {
-    lineRefs.current  = [];
-    wordRefs.current  = [];
-    prevActive.current = { line: -1, word: -1 };
-    scrollerRef.current?.jump(0);
+    const id = currentTrack?.id ?? null;
+    if (prevTrackId.current !== undefined && prevTrackId.current !== id) {
+      lineRefs.current = [];
+      wordRefs.current = [];
+      prevActive.current = { line: -1, word: -1 };
+      scrollerRef.current?.jump(0);
+    }
+    prevTrackId.current = id;
   }, [currentTrack?.id]);
 
   // Imperative tracker — subscribes directly to the store, zero React re-renders per tick.