feat(player): Now Playing overlay + synced lyrics panel (A1/A11, §6.7)

Full-screen overlay expanded from the persistent player: big artwork or a
Lyrics tab. Synced (LRC) lyrics highlight the active line by playback position,
auto-scroll, and seek on tap; plain text is the fallback; missing lyrics offers
a re-fetch. Adds lyrics endpoint + LRC parser + isNowPlayingOpen player state.

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

src/lib/lrc.ts

@@ -0,0 +1,50 @@
+/** One parsed LRC line: a timestamp (seconds) and its text. */
+export interface LrcLine {
+  time: number;
+  text: string;
+}
+
+const TAG = /\[(\d{1,2}):(\d{2})(?:[.:](\d{1,3}))?\]/g;
+
+/**
+ * Parse an LRC string into time-sorted lines. A single source line may carry
+ * several timestamps (repeated lyric) — each becomes its own entry. Metadata
+ * tags (`[ar:…]`, `[ti:…]`) and untimed lines are skipped; empty-text lines are
+ * kept so the active-line highlight advances through instrumental gaps.
+ */
+export function parseLrc(lrc: string): LrcLine[] {
+  const lines: LrcLine[] = [];
+  for (const raw of lrc.split(/\r?\n/)) {
+    const stamps = [...raw.matchAll(TAG)];
+    if (stamps.length === 0) continue;
+    const text = raw.replace(/\[[^\]]*\]/g, '').trim();
+    for (const m of stamps) {
+      const min = Number(m[1]);
+      const sec = Number(m[2]);
+      const frac = m[3] ? Number(m[3].padEnd(3, '0')) / 1000 : 0;
+      lines.push({ time: min * 60 + sec + frac, text });
+    }
+  }
+  return lines.sort((a, b) => a.time - b.time);
+}
+
+/**
+ * Index of the active line for a playback position (seconds) — the last line
+ * whose timestamp has passed. Returns -1 before the first line. Assumes `lines`
+ * is time-sorted (as `parseLrc` returns).
+ */
+export function activeLineIndex(lines: LrcLine[], position: number): number {
+  let lo = 0;
+  let hi = lines.length - 1;
+  let result = -1;
+  while (lo <= hi) {
+    const mid = (lo + hi) >> 1;
+    if (lines[mid].time <= position) {
+      result = mid;
+      lo = mid + 1;
+    } else {
+      hi = mid - 1;
+    }
+  }
+  return result;
+}