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/endpoints/lyrics.ts

@@ -0,0 +1,49 @@
+import { api } from '../index';
+import type { Lyrics } from '../types';
+
+// `GET /tracks/{id}/lyrics` — cached lyrics for the Now Playing panel (§6.7).
+// A miss is a normal 200 with `status: 'not_found'`, not an error.
+
+interface RawLyrics {
+  track_id: string;
+  status: string;
+  source: string | null;
+  synced: string | null;
+  plain: string | null;
+  synced_available: boolean;
+}
+
+const toLyrics = (r: RawLyrics): Lyrics => ({
+  trackId: r.track_id,
+  status: r.status as Lyrics['status'],
+  source: r.source ?? undefined,
+  synced: r.synced ?? undefined,
+  plain: r.plain ?? undefined,
+  syncedAvailable: r.synced_available,
+});
+
+export const lyricsApi = api.injectEndpoints({
+  endpoints: (build) => ({
+    getLyrics: build.query<Lyrics, string>({
+      query: (trackId) => `/tracks/${trackId}/lyrics`,
+      transformResponse: (raw: RawLyrics) => toLyrics(raw),
+    }),
+    refetchLyrics: build.mutation<Lyrics, string>({
+      query: (trackId) => ({
+        url: `/tracks/${trackId}/lyrics/refetch`,
+        method: 'POST',
+      }),
+      transformResponse: (raw: RawLyrics) => toLyrics(raw),
+      // Push the fresh result straight into the getLyrics cache for this track.
+      async onQueryStarted(trackId, { dispatch, queryFulfilled }) {
+        const { data } = await queryFulfilled;
+        dispatch(
+          lyricsApi.util.upsertQueryData('getLyrics', trackId, data),
+        );
+      },
+    }),
+  }),
+  overrideExisting: false,
+});
+
+export const { useGetLyricsQuery, useRefetchLyricsMutation } = lyricsApi;

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;