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/app/AppShell.tsx

@@ -1,3 +1,4 @@
+import { probeEntityRatingSupport } from '../api/subsonicStarRating';
 import { getMusicFolders } from '../api/subsonicLibrary';
 import React, { Suspense, useCallback, useEffect, useRef, useState } from 'react';
 import { useLocation, useNavigate } from 'react-router-dom';
@@ -37,7 +38,6 @@ import { useOrbitStore } from '../store/orbitStore';
 import { IS_LINUX, IS_MACOS, IS_WINDOWS } from '../utils/platform';
 import { useConnectionStatus } from '../hooks/useConnectionStatus';
 import { useAuthStore } from '../store/authStore';
-import { probeEntityRatingSupport } from '../api/subsonic';
 import { useOfflineStore } from '../store/offlineStore';
 import { usePlayerStore } from '../store/playerStore';
 import { useThemeStore } from '../store/themeStore';

src/app/TauriEventBridge.tsx

@@ -13,7 +13,7 @@ import { showToast } from '../utils/toast';
 import { endOrbitSession, leaveOrbitSession } from '../utils/orbit';
 import { useOrbitStore } from '../store/orbitStore';
 import { useAuthStore } from '../store/authStore';
-import { search as subsonicSearch } from '../api/subsonic';
+import { search as subsonicSearch } from '../api/subsonicSearch';
 import i18n from '../i18n';
 import { switchActiveServer } from '../utils/switchActiveServer';
 import { usePlayerStore } from '../store/playerStore';