refactor(api): F.50 — extract 7 small domain modules (#615)

Seven domain-eng splits peel ~200 LOC of read endpoints out of
`api/subsonic.ts`:

- `subsonicStreamUrl.ts` — `buildStreamUrl`, `coverArtCacheKey`,
  `buildCoverArtUrl`, `buildDownloadUrl` (token-signed URL builders
  for the four /rest endpoints we hand to the browser).
- `subsonicStarRating.ts` — `getStarred`, `star`, `unstar`,
  `setRating`, `probeEntityRatingSupport`. `setRating` still triggers
  the lazy `navidromeBrowse` cache invalidation; the same-folder
  lazy import path is preserved.
- `subsonicSearch.ts` — `search`, `searchSongsPaged`.
- `subsonicScrobble.ts` — `scrobbleSong`, `reportNowPlaying`,
  `getNowPlaying`.
- `subsonicAlbumInfo.ts` — `getAlbumInfo2`.
- `subsonicLyrics.ts` — `getLyricsBySongId`.
- `subsonicGenres.ts` — `getGenres`, `getAlbumsByGenre`.

63 external call sites migrated to direct imports. Four `vi.mock`
targets in the store-level tests pointed at `../api/subsonic` and
were updated to the new module paths.

Pure code-move. subsonic.ts: 762 → 561 LOC (−201).

src/api/subsonicLyrics.ts

@@ -0,0 +1,23 @@
+import { api } from './subsonicClient';
+import type { SubsonicStructuredLyrics } from './subsonicTypes';
+
+/**
+ * Fetches structured lyrics from the server's embedded tags via the
+ * OpenSubsonic `getLyricsBySongId` endpoint. Returns null when the
+ * server doesn't support the endpoint or the track has no embedded lyrics.
+ * Prefers synced lyrics over plain when both are present.
+ */
+export async function getLyricsBySongId(id: string): Promise<SubsonicStructuredLyrics | null> {
+  try {
+    const data = await api<{ lyricsList: { structuredLyrics?: SubsonicStructuredLyrics[] } }>(
+      'getLyricsBySongId.view',
+      { id },
+    );
+    const list = data.lyricsList?.structuredLyrics;
+    if (!list || list.length === 0) return null;
+    return list.find(l => l.synced || l.issynced) ?? list[0];
+  } catch {
+    // Server doesn't support the endpoint or track has no embedded lyrics
+    return null;
+  }
+}