refactor(api): F.49 — extract library + artists + ratings (#614)

Three domain-eng modules peel ~316 LOC of fetch/mapping out of
`api/subsonic.ts`:

- `subsonicLibrary.ts` — browse + random + per-song fetch
  (`getMusicDirectory`, `getMusicIndexes`, `getMusicFolders`,
  `getRandomAlbums`, `getAlbumList`, `getRandomSongs`,
  `getRandomSongsFiltered`, `getSong`, `getAlbum`,
  `filterSongsToActiveLibrary`, `similarSongsRequestCount`, plus
  the private `albumIdsInActiveLibraryScope` cache).
- `subsonicArtists.ts` — artist endpoints (`getArtists`, `getArtist`,
  `getArtistInfo`, `getTopSongs`, `getSimilarSongs2`,
  `getSimilarSongs`). Uses Library's `filterSongsToActiveLibrary` and
  `similarSongsRequestCount` for the per-library scoping fallback.
- `subsonicRatings.ts` — `parseSubsonicEntityStarRating` parser plus
  `prefetchArtistUserRatings` and `prefetchAlbumUserRatings` workers
  with the shared 7-min cache. Calls back into Library/Artists for
  the per-id fetch.

51 external call sites migrated to direct imports. No re-export
shims in `subsonic.ts`. Statistics endpoints still in `subsonic.ts`
keep their `RATING_CACHE_TTL` constant locally (same 7-min window).

subsonic.ts: 1078 → 762 LOC (−316).

src/store/nextAction.ts

@@ -1,5 +1,5 @@
+import { getSimilarSongs2, getTopSongs } from '../api/subsonicArtists';
 import { invoke } from '@tauri-apps/api/core';
-import { getSimilarSongs2, getTopSongs } from '../api/subsonic';
 import { buildInfiniteQueueCandidates } from '../utils/buildInfiniteQueueCandidates';
 import { songToTrack } from '../utils/songToTrack';
 import { useAuthStore } from './authStore';

src/store/offlineStore.ts

@@ -1,8 +1,10 @@
+import { getAlbum } from '../api/subsonicLibrary';
+import { getArtist } from '../api/subsonicArtists';
 import type { SubsonicSong } from '../api/subsonicTypes';
 import { create } from 'zustand';
 import { persist, createJSONStorage } from 'zustand/middleware';
 import { invoke } from '@tauri-apps/api/core';
-import { buildStreamUrl, getArtist, getAlbum } from '../api/subsonic';
+import { buildStreamUrl } from '../api/subsonic';
 import { useAuthStore } from './authStore';
 import { showToast } from '../utils/toast';
 import { useOfflineJobStore, cancelledDownloads } from './offlineJobStore';

src/store/resumeAction.ts

@@ -1,6 +1,6 @@
+import { getSong } from '../api/subsonicLibrary';
 import { invoke } from '@tauri-apps/api/core';
 import { estimateLivePosition } from '../api/orbit';
-import { getSong } from '../api/subsonic';
 import { setDeferHotCachePrefetch } from '../utils/hotCacheGate';
 import { resolvePlaybackUrl } from '../utils/resolvePlaybackUrl';
 import { resolveReplayGainDb } from '../utils/resolveReplayGainDb';