refactor(api): F.51 — extract playlists + play queue + radio + statistics (#616)

Four more domain-eng modules out of `api/subsonic.ts`:

- `subsonicPlaylists.ts` — `getPlaylists`/`getPlaylist`/`createPlaylist`
  /`updatePlaylist`/`updatePlaylistMeta`/`uploadPlaylistCoverArt`/
  `uploadArtistImage`/`deletePlaylist`. The two upload helpers use
  the Tauri-side CORS bypass (`upload_playlist_cover` /
  `upload_artist_image`).
- `subsonicPlayQueue.ts` — `getPlayQueue`/`savePlayQueue`.
- `subsonicRadio.ts` — Internet Radio CRUD (4 fns), Tauri-side cover
  art ops (3 fns), RadioBrowser search/top + `fetchUrlBytes`.
- `subsonicStatistics.ts` — `fetchStatisticsLibraryAggregates`/
  `fetchStatisticsOverview`/`fetchStatisticsFormatSample` with their
  three per-server-folder caches and the `statisticsPageCacheKey`
  helper. `STATS_CACHE_TTL` renamed from the misleadingly-shared
  `RATING_CACHE_TTL` constant.

Also: drop ~20 now-unused type/value imports from `subsonic.ts`, trim
three orphan jsdoc comments left behind by earlier slices, fix four
`vi.mock` targets (`queueSync`, `playerStore.persistence`) plus the
dynamic `await import('../api/subsonic')` calls in `ContextMenu.tsx`
to point at the new module paths.

Pure code-move. subsonic.ts: 561 → 144 LOC (−417). What's left:
ping/pingWithCredentials/probeInstantMix/scheduleInstantMixProbe +
internal `apiWithCredentials`/`restBaseFromUrl`.

src/pages/Playlists.tsx

@@ -1,3 +1,4 @@
+import { deletePlaylist, getPlaylist, updatePlaylist } from '../api/subsonicPlaylists';
 import { getGenres } from '../api/subsonicGenres';
 import { buildCoverArtUrl, coverArtCacheKey } from '../api/subsonicStreamUrl';
 import { filterSongsToActiveLibrary } from '../api/subsonicLibrary';
@@ -6,7 +7,6 @@ import { songToTrack } from '../utils/songToTrack';
 import React, { useEffect, useState, useRef, useCallback, useMemo } from 'react';
 import { useNavigate } from 'react-router-dom';
 import { ListMusic, Play, Plus, Trash2, CheckSquare2, Check, Clock3, Sparkles, Pencil } from 'lucide-react';
-import { deletePlaylist, getPlaylist, updatePlaylist } from '../api/subsonic';
 import { usePlayerStore } from '../store/playerStore';
 import { usePlaylistStore } from '../store/playlistStore';
 import { useAuthStore } from '../store/authStore';