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/api/types.ts

@@ -192,6 +192,17 @@ export interface DuplicateGroup {
   tracks: Track[];
 }
 
+/** Cached lyrics for a track (§6.7). `not_found` is a normal state, not an
+ * error. `synced` is raw LRC (client parses timestamps); `plain` is fallback. */
+export interface Lyrics {
+  trackId: string;
+  status: 'found' | 'not_found' | 'pending';
+  source?: string;
+  synced?: string;
+  plain?: string;
+  syncedAvailable: boolean;
+}
+
 export interface User {
   id: string;
   username: string;