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/store/loudnessRefresh.test.ts

@@ -38,7 +38,7 @@ const hoisted = vi.hoisted(() => {
 });
 
 vi.mock('@tauri-apps/api/core', () => ({ invoke: hoisted.invokeMock }));
-vi.mock('../api/subsonic', () => ({ buildStreamUrl: hoisted.buildStreamUrlMock }));
+vi.mock('../api/subsonicStreamUrl', () => ({ buildStreamUrl: hoisted.buildStreamUrlMock }));
 vi.mock('../utils/redactSubsonicUrl', () => ({ redactSubsonicUrlForLog: hoisted.redactMock }));
 vi.mock('./authStore', () => ({ useAuthStore: { getState: () => hoisted.auth } }));
 vi.mock('./playerStore', () => ({