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/PlaylistDetail.tsx

@@ -1,3 +1,6 @@
+import { buildDownloadUrl, coverArtCacheKey, buildCoverArtUrl } from '../api/subsonicStreamUrl';
+import { setRating, star, unstar } from '../api/subsonicStarRating';
+import { search } from '../api/subsonicSearch';
 import { getRandomSongs, filterSongsToActiveLibrary } from '../api/subsonicLibrary';
 import type { SubsonicPlaylist, SubsonicSong } from '../api/subsonicTypes';
 import { songToTrack } from '../utils/songToTrack';
@@ -7,7 +10,7 @@ import { useParams, useNavigate, useLocation } from 'react-router-dom';
 import { ChevronDown, ChevronLeft, ChevronRight, Play, ListPlus, Trash2, Search, X, Loader2, Plus, GripVertical, Star, RefreshCw, Shuffle, Heart, HardDriveDownload, Check, Pencil, Globe, Lock, Camera, Download, FileUp, RotateCcw, Sparkles, Square, AudioLines } from 'lucide-react';
 import { useTracklistColumns, type ColDef } from '../utils/useTracklistColumns';
 import { AddToPlaylistSubmenu } from '../components/ContextMenu';
-import { getPlaylist, updatePlaylist, updatePlaylistMeta, uploadPlaylistCoverArt, search, setRating, star, unstar, buildDownloadUrl } from '../api/subsonic';
+import { getPlaylist, updatePlaylist, updatePlaylistMeta, uploadPlaylistCoverArt } from '../api/subsonic';
 import { usePlayerStore } from '../store/playerStore';
 import { useShallow } from 'zustand/react/shallow';
 import { usePlaylistStore } from '../store/playlistStore';
@@ -25,7 +28,6 @@ import { readTextFile } from '@tauri-apps/plugin-fs';
 import { useZipDownloadStore } from '../store/zipDownloadStore';
 import { useDragDrop } from '../contexts/DragDropContext';
 import CachedImage, { useCachedUrl } from '../components/CachedImage';
-import { coverArtCacheKey, buildCoverArtUrl } from '../api/subsonic';
 import { useTranslation } from 'react-i18next';
 import { showToast } from '../utils/toast';
 import { formatHumanHoursMinutes } from '../utils/formatHumanDuration';