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/subsonicGenres.ts

@@ -0,0 +1,23 @@
+import { api, libraryFilterParams } from './subsonicClient';
+import type { SubsonicAlbum, SubsonicGenre } from './subsonicTypes';
+
+export async function getGenres(): Promise<SubsonicGenre[]> {
+  const data = await api<{ genres: { genre: SubsonicGenre | SubsonicGenre[] } }>('getGenres.view');
+  const raw = data.genres?.genre;
+  if (!raw) return [];
+  return Array.isArray(raw) ? raw : [raw];
+}
+
+export async function getAlbumsByGenre(genre: string, size = 50, offset = 0): Promise<SubsonicAlbum[]> {
+  const data = await api<{ albumList2: { album: SubsonicAlbum | SubsonicAlbum[] } }>('getAlbumList2.view', {
+    type: 'byGenre',
+    genre,
+    size,
+    offset,
+    _t: Date.now(),
+    ...libraryFilterParams(),
+  });
+  const raw = data.albumList2?.album;
+  if (!raw) return [];
+  return Array.isArray(raw) ? raw : [raw];
+}