perf(web): defer lyrics prefetch so it doesn't slow playback start

prefetchLyrics now fires ~1.5s after a song change instead of immediately,
so the lrclib requests no longer compete with audio-stream buffering when a
new track starts. Timer is cleared on rapid song changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/player/nowplaying.js

@@ -100,6 +100,7 @@ export function switchNpTab(tab) {
 let _lyricsLines = [];
 let _lyricsSongId = null;
 let _lyricsLoadToken = 0;
+let _prefetchTimer = null;
 const _lyricsCache = new Map(); // "artist|title" → result object (null = not found)
 
 const LYRICS_LOOKAHEAD_MS = 200;
@@ -283,8 +284,11 @@ export function onNpSongChange(songId) {
   _lyricsLines = [];
   const content = document.getElementById('np-lyrics-content');
   if(content) content.innerHTML = '';
-  // Warm the cache the moment the song changes, so the Lyrics tab opens instantly.
-  prefetchLyrics(S.queue.find(x => x.id === songId) || S.queue[S.qIdx]);
+  // Warm the cache shortly AFTER the track starts — deferred so the lyrics requests
+  // don't compete with the audio stream and delay playback start.
+  clearTimeout(_prefetchTimer);
+  const song = S.queue.find(x => x.id === songId) || S.queue[S.qIdx];
+  _prefetchTimer = setTimeout(() => prefetchLyrics(song), 1500);
   if(S.npLyricsMode) _loadNpLyrics(songId);
 }