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/api/subsonic.contract.test.ts

@@ -11,15 +11,8 @@
  * mocking and are not in this PR.
  */
 import { beforeEach, describe, expect, it } from 'vitest';
-import {
-  buildCoverArtUrl,
-  buildDownloadUrl,
-  buildStreamUrl,
-  coverArtCacheKey,
-  getClient,
-  libraryFilterParams,
-  parseSubsonicEntityStarRating,
-} from './subsonic';
+import { buildCoverArtUrl, buildDownloadUrl, buildStreamUrl, coverArtCacheKey, parseSubsonicEntityStarRating } from './subsonic';
+import { getClient, libraryFilterParams } from './subsonicClient';
 import { useAuthStore } from '@/store/authStore';
 import { resetAuthStore } from '@/test/helpers/storeReset';