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/pages/RandomMix.tsx

@@ -1,8 +1,9 @@
+import { star, unstar } from '../api/subsonicStarRating';
+import { getGenres } from '../api/subsonicGenres';
 import type { SubsonicSong, SubsonicGenre } from '../api/subsonicTypes';
 import { RANDOM_MIX_SIZE_OPTIONS } from '../store/authStoreDefaults';
 import { songToTrack } from '../utils/songToTrack';
 import React, { useEffect, useMemo, useState } from 'react';
-import { getGenres, star, unstar } from '../api/subsonic';
 import { usePlayerStore } from '../store/playerStore';
 import { usePreviewStore } from '../store/previewStore';
 import { useAuthStore } from '../store/authStore';