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/DeviceSync.tsx

@@ -1,3 +1,4 @@
+import type { SubsonicSong, SubsonicAlbum, SubsonicPlaylist, SubsonicArtist } from '../api/subsonicTypes';
 import React, { useEffect, useState, useRef, useCallback, useMemo } from 'react';
 import { invoke } from '@tauri-apps/api/core';
 import { listen } from '@tauri-apps/api/event';
@@ -11,11 +12,7 @@ import CustomSelect from '../components/CustomSelect';
 import { useTranslation } from 'react-i18next';
 import { useDeviceSyncStore, DeviceSyncSource } from '../store/deviceSyncStore';
 import { useDeviceSyncJobStore } from '../store/deviceSyncJobStore';
-import {
-  getPlaylists, getAlbumList, getArtists, getAlbum, getPlaylist, getArtist,
-  buildDownloadUrl, search as searchSubsonic,
-  SubsonicSong, SubsonicAlbum, SubsonicPlaylist, SubsonicArtist,
-} from '../api/subsonic';
+import { getPlaylists, getAlbumList, getArtists, getAlbum, getPlaylist, getArtist, buildDownloadUrl, search as searchSubsonic } from '../api/subsonic';
 import { showToast } from '../utils/toast';
 import { IS_WINDOWS } from '../utils/platform';
 
@@ -598,7 +595,7 @@ export default function DeviceSync() {
     setPreSyncOpen(true);
 
     try {
-      const { getClient } = await import('../api/subsonic');
+      const { getClient } = await import('../api/subsonicClient');
       const { baseUrl, params } = getClient();
       const payload = await invoke<{
         addBytes: number; addCount: number; delBytes: number; delCount: number; availableBytes: number; tracks: SubsonicSong[];