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/store/authStoreTypes.ts

@@ -1,4 +1,4 @@
-import type { EntityRatingSupportLevel } from '../api/subsonic';
+import type { EntityRatingSupportLevel } from '../api/subsonicTypes';
 import type {
   InstantMixProbeResult,
   SubsonicServerIdentity,

src/store/offlineStore.ts

@@ -1,8 +1,8 @@
+import type { SubsonicSong } from '../api/subsonicTypes';
 import { create } from 'zustand';
 import { persist, createJSONStorage } from 'zustand/middleware';
 import { invoke } from '@tauri-apps/api/core';
 import { buildStreamUrl, getArtist, getAlbum } from '../api/subsonic';
-import type { SubsonicSong } from '../api/subsonic';
 import { useAuthStore } from './authStore';
 import { showToast } from '../utils/toast';
 import { useOfflineJobStore, cancelledDownloads } from './offlineJobStore';

src/store/playerStoreTypes.ts

@@ -1,4 +1,4 @@
-import type { InternetRadioStation } from '../api/subsonic';
+import type { InternetRadioStation } from '../api/subsonicTypes';
 import type { PlaybackSourceKind } from '../utils/resolvePlaybackUrl';
 
 export interface Track {

src/store/playlistStore.ts

@@ -1,7 +1,7 @@
+import type { SubsonicPlaylist } from '../api/subsonicTypes';
 import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
-import { getPlaylists, createPlaylist as apiCreatePlaylist, SubsonicPlaylist } from '../api/subsonic';
-
+import { getPlaylists, createPlaylist as apiCreatePlaylist } from '../api/subsonic';
 interface PlaylistStore {
   recentIds: string[];
   playlists: SubsonicPlaylist[];