fix: lyrics sync offset tag + 200ms lookahead

- Parse [offset:N] metadata from LRC files (was silently ignored, caused ~1s drift)
- Apply 200ms lookahead so active line highlights slightly before it's sung

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui.js

@@ -474,12 +474,19 @@ export function switchNpTab(tab) {
 let _lyricsLines = [];
 let _lyricsSongId = null;
 
+const LYRICS_LOOKAHEAD_MS = 200; // highlight line this many ms before its timestamp
+
 function _parseLrc(text) {
+  let offsetMs = 0;
   const lines = [];
   for(const raw of text.split('\n')) {
+    const offM = raw.match(/^\[offset:\s*([+-]?\d+)\s*\]/i);
+    if(offM) { offsetMs = parseInt(offM[1]); continue; }
     const m = raw.match(/^\[(\d+):(\d+(?:\.\d+)?)\](.*)/);
     if(m) lines.push({ start: Math.round((parseInt(m[1])*60 + parseFloat(m[2]))*1000), value: m[3].trim() });
   }
+  // [offset:N] shifts all timestamps by N ms (positive = lines appear later)
+  if(offsetMs) lines.forEach(l => l.start = Math.max(0, l.start + offsetMs));
   return lines.sort((a,b) => a.start - b.start).filter(l => l.value);
 }
 
@@ -559,9 +566,10 @@ async function _loadNpLyrics(songId) {
 
 export function updateLyricsHighlight(currentMs) {
   if(!_lyricsLines.length || !S.npLyricsMode) return;
+  const adjusted = currentMs + LYRICS_LOOKAHEAD_MS;
   let activeIdx = -1;
   for(let i = 0; i < _lyricsLines.length; i++) {
-    if(_lyricsLines[i].start <= currentMs) activeIdx = i;
+    if(_lyricsLines[i].start <= adjusted) activeIdx = i;
     else break;
   }
   _lyricsLines.forEach((line, i) => {