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/QueuePanel.tsx

@@ -1,3 +1,4 @@
+import { buildCoverArtUrl, coverArtCacheKey } from '../api/subsonicStreamUrl';
 import { getAlbum } from '../api/subsonicLibrary';
 import type { SubsonicPlaylist } from '../api/subsonicTypes';
 import { registerQueueListScrollTopReader, consumePendingQueueListScrollTop } from '../store/queueUndo';
@@ -11,7 +12,7 @@ import OrbitGuestQueue from './OrbitGuestQueue';
 import OrbitQueueHead from './OrbitQueueHead';
 import HostApprovalQueue from './HostApprovalQueue';
 import { Play, Music, Star, X, Trash2, Save, FolderOpen, Shuffle, Infinity, Waves, MicVocal, ListMusic, Check, ListPlus, MoveRight, Radio, HardDrive, ChevronDown, Info, Share2 } from 'lucide-react';
-import { buildCoverArtUrl, coverArtCacheKey, getPlaylists, getPlaylist, updatePlaylist, deletePlaylist } from '../api/subsonic';
+import { getPlaylists, getPlaylist, updatePlaylist, deletePlaylist } from '../api/subsonic';
 import { usePlaylistStore } from '../store/playlistStore';
 import { useCachedUrl } from './CachedImage';
 import { useTranslation } from 'react-i18next';