refactor(api): F.48 — extract subsonic types + client primitives (#613)

First Phase F slice. Splits the 1333-LOC `api/subsonic.ts` along its
two most obvious axes:

- `subsonicTypes.ts` — all ~24 exported interfaces + type aliases
  (album/song/artist/playlist/directory/genre/now-playing/radio,
  random-songs filters, three statistics shapes, search + starred
  results, AlbumInfo, structured-lyrics types, etc.) plus the
  `RADIO_PAGE_SIZE` constant.
- `subsonicClient.ts` — token-auth + `getClient` + `api<T>()` +
  `libraryFilterParams` + `secureRandomSalt` / `getAuthParams` /
  `SUBSONIC_CLIENT`. The credential-bearing API helpers
  (`pingWithCredentials`, `apiWithCredentials`, `restBaseFromUrl`,
  `probeInstantMixWithCredentials`) stay in `subsonic.ts` for now —
  they could move into the client module in a follow-up.

66 external call sites migrated to direct imports from the new
modules (no re-export shims in `subsonic.ts`). Pure code-move;
contract test stays green.

subsonic.ts: 1333 → 1078 LOC (−255).

src/pages/PlaylistDetail.tsx

@@ -1,3 +1,4 @@
+import type { SubsonicPlaylist, SubsonicSong } from '../api/subsonicTypes';
 import { songToTrack } from '../utils/songToTrack';
 import React, { useEffect, useState, useRef, useCallback, useMemo } from 'react';
 import { createPortal } from 'react-dom';
@@ -5,11 +6,7 @@ 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,
-  getRandomSongs, buildDownloadUrl, filterSongsToActiveLibrary, SubsonicPlaylist, SubsonicSong,
-} from '../api/subsonic';
+import { getPlaylist, updatePlaylist, updatePlaylistMeta, uploadPlaylistCoverArt, search, setRating, star, unstar, getRandomSongs, buildDownloadUrl, filterSongsToActiveLibrary } from '../api/subsonic';
 import { usePlayerStore } from '../store/playerStore';
 import { useShallow } from 'zustand/react/shallow';
 import { usePlaylistStore } from '../store/playlistStore';