refactor(api): pull feature-resident subsonic clients into lib/api

Move the 5 subsonic api modules that M3 had co-located into features
(subsonicAlbumInfo/Artists/Playlists/Radio/Statistics) into src/lib/api/, the
feature-free infra layer, and drop their feature-barrel re-exports. Consumers
now import these protocol calls directly from @/lib/api/subsonic*; the feature
barrels export only domain UI/hooks/state.

This is the consistent api placement (plan §10.3, revised: ALL subsonic protocol
clients are feature-free infra, not per-feature) and it kills the documented
api-induced feature<->feature cycles: offline->artist/playlist (getArtist/
getPlaylist*), album->artist (getArtistInfo), deviceSync/nowPlaying/orbit->*.
Remaining artist<->album refs are UI-only (OpenArtistRefInline/coerceOpenArtist
Refs) and offline->playlist is store-level (usePlaylistStore) — both deeper,
tracked for M5, not api placement.

Pure move: import specifiers + vi.mock/spy targets repointed; no behaviour
change. tsc 0, lint 0/0, full suite 319 files / 2353 tests green.

Tooling note: barrel-imported and dynamic-import api symbols were split out of
@/features/* to @/lib/api/* (tsc-driven); 12 vi.mock barrels retargeted to the
deep lib module (mock-collapse sweep), AlbumHeader's UI mock left untouched.

src/features/offline/store/offlineStore.ts

@@ -1,7 +1,7 @@
 import { libraryUpsertSongsFromApi } from '@/lib/api/library';
 import { buildStreamUrl } from '@/lib/api/subsonicStreamUrl';
 import { getAlbum } from '@/lib/api/subsonicLibrary';
-import { getArtist } from '@/features/artist';
+import { getArtist } from '@/lib/api/subsonicArtists';
 import type { SubsonicSong } from '@/lib/api/subsonicTypes';
 import { create } from 'zustand';
 import { persist, createJSONStorage } from 'zustand/middleware';