feat(lyrics): prefetch on play + smooth measured-position autoscroll

Warm the lrclib cache the moment a track starts (Android PlayerController
onMediaItemTransition; web onNpSongChange) so opening the Lyrics tab is
instant. Extract a DOM-free fetch helper on web for reuse.

Rewrite Android lyrics autoscroll to center on the active line's measured
position in a LazyColumn instead of a fixed line-height estimate — removes
stutter and the downward drift on wrapped lines; active line sits at a
~40%-from-top reading anchor.

CHANGELOG: fold into 1.0; drop the now-fixed lyrics known-issue note.

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

src/player/nowplaying.js

@@ -145,7 +145,6 @@ async function _loadNpLyrics(songId) {
   }
   _lyricsSongId = songId;
   _lyricsLines = [];
-  content.innerHTML = '<div style="text-align:center;padding:60px 0;color:rgba(255,255,255,.3);font-size:14px">Lade Lyrics…</div>';
 
   const token = ++_lyricsLoadToken;
   const s      = S.queue[S.qIdx];
@@ -155,6 +154,28 @@ async function _loadNpLyrics(songId) {
   const dur    = s?.duration || 0;
   const cacheKey = `${artist}|${title}`;
 
+  // Cache hit (warmed on play, or from a previous view) — render instantly, no placeholder flash
+  if(_lyricsCache.has(cacheKey)) {
+    content.innerHTML = '';
+    _showLyricsResult(content, _lyricsCache.get(cacheKey));
+    return;
+  }
+
+  content.innerHTML = '<div style="text-align:center;padding:60px 0;color:rgba(255,255,255,.3);font-size:14px">Lade Lyrics…</div>';
+  const result = await _fetchLyricsData(artist, title, album, dur);
+  if(token !== _lyricsLoadToken) return;
+  content.innerHTML = '';
+  _showLyricsResult(content, result);
+}
+
+/**
+ * Pure lrclib fetch cascade — no DOM. Populates _lyricsCache and returns the result.
+ * Shared by the Lyrics tab and by prefetchLyrics() so opening Lyrics feels instant.
+ */
+async function _fetchLyricsData(artist, title, album, dur) {
+  const cacheKey = `${artist}|${title}`;
+  if(_lyricsCache.has(cacheKey)) return _lyricsCache.get(cacheKey);
+
   const SIG = () => AbortSignal.timeout(6000);
   const safeJson = async (promise) => { try { const r = await promise; return r.ok ? r.json() : null; } catch { return null; } };
   const bestMatch = (candidates, durSecs) => {
@@ -166,19 +187,12 @@ async function _loadNpLyrics(songId) {
     return Math.abs(pick.duration - durSecs) <= 5 ? pick : (synced[0] || candidates[0]);
   };
 
-  // Cache hit — instant
-  if(_lyricsCache.has(cacheKey)) {
-    const cached = _lyricsCache.get(cacheKey);
-    content.innerHTML = '';
-    _showLyricsResult(content, cached);
-    return;
-  }
+  let result = null;
 
   // 1a. lrclib exact match with duration
   const exactP = new URLSearchParams({ artist_name: artist, track_name: title });
   if(album) exactP.set('album_name', album);
   if(dur)   exactP.set('duration', dur);
-  let result = null;
   try {
     const r = await fetch(`https://lrclib.net/api/get?${exactP}`, { signal: SIG() });
     if(r.status === 200) {
@@ -199,8 +213,6 @@ async function _loadNpLyrics(songId) {
     } catch {}
   }
 
-  if(token !== _lyricsLoadToken) return;
-
   // 2+3. lrclib search — artist+title and title-only in parallel (2 requests, not 3)
   if(!result) {
     const artistP = new URLSearchParams({ track_name: title });
@@ -214,12 +226,15 @@ async function _loadNpLyrics(songId) {
     result = bestMatch(artistData, dur) ?? bestMatch(titleData, dur);
   }
 
-  if(token !== _lyricsLoadToken) return;
-
-  if(token !== _lyricsLoadToken) return;
   _lyricsCache.set(cacheKey, result ?? null);
-  content.innerHTML = '';
-  _showLyricsResult(content, result);
+  return result ?? null;
+}
+
+/** Warm the lyrics cache when a track starts, so opening the Lyrics tab is instant. */
+function prefetchLyrics(song) {
+  if(!song || !song.title) return;
+  const artist = (song.artist || '').split(/[/,]/)[0].trim();
+  _fetchLyricsData(artist, song.title, song.album || '', song.duration || 0).catch(() => {});
 }
 
 function _showLyricsResult(content, result) {
@@ -268,6 +283,8 @@ 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]);
   if(S.npLyricsMode) _loadNpLyrics(songId);
 }