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/test/helpers/factories.ts

@@ -5,10 +5,9 @@
  * the fields the test cares about. Keeps tests focused on behaviour rather
  * than on assembling boilerplate.
  */
+import type { SubsonicSong } from '@/api/subsonicTypes';
 import type { ServerProfile } from '@/store/authStoreTypes';
 import type { Track } from '@/store/playerStoreTypes';
-import type { SubsonicSong } from '@/api/subsonic';
-
 let trackCounter = 0;
 let songCounter = 0;
 let serverCounter = 0;

src/test/mocks/subsonic.ts

@@ -18,7 +18,7 @@
  * Realistic shape matters more than perfect coverage — these fixtures
  * mirror what Navidrome actually returns for common queries.
  */
-import type { SubsonicSong, SubsonicAlbum, SubsonicPlaylist } from '@/api/subsonic';
+import type { SubsonicSong, SubsonicAlbum, SubsonicPlaylist } from '@/api/subsonicTypes';
 import { makeSubsonicSong } from '@/test/helpers/factories';
 
 export const sampleSubsonicSong: SubsonicSong = makeSubsonicSong({