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/components/ContextMenu.tsx

@@ -1,3 +1,5 @@
+import { buildDownloadUrl } from '../api/subsonicStreamUrl';
+import { star, unstar, setRating } from '../api/subsonicStarRating';
 import { getAlbum } from '../api/subsonicLibrary';
 import { getSimilarSongs2, getSimilarSongs, getTopSongs, getArtist } from '../api/subsonicArtists';
 import type { SubsonicAlbum, SubsonicArtist, SubsonicPlaylist } from '../api/subsonicTypes';
@@ -17,7 +19,7 @@ import StarRating from './StarRating';
 import { lastfmLoveTrack, lastfmUnloveTrack } from '../api/lastfm';
 import { usePlayerStore } from '../store/playerStore';
 import { useShallow } from 'zustand/react/shallow';
-import { star, unstar, buildDownloadUrl, getPlaylists, getPlaylist, updatePlaylist, setRating } from '../api/subsonic';
+import { getPlaylists, getPlaylist, updatePlaylist } from '../api/subsonic';
 import { useNavigate } from 'react-router-dom';
 import { useAuthStore } from '../store/authStore';
 import { useDownloadModalStore } from '../store/downloadModalStore';