refactor(api): co-locate server-protocol clients into lib/api

Move the Subsonic + Navidrome protocol clients and the library IPC facade
(subsonic*, navidrome*, library.ts + tests) from src/api/ into src/lib/api/,
the feature-free infra layer (plan M4, decision §10.3: shared client core → lib/).
Pure move: deep-path import specifiers @/api/* -> @/lib/api/* across ~150
consumers; no behaviour change. Domain REST (lyrics/events) and cover/analysis
infra stay in src/api/ pending their own M4 placement.

tsc 0, lint 0/0, full suite 319 files / 2353 tests green.

src/utils/share/enqueueShareSearchPayload.test.ts

@@ -22,7 +22,7 @@ const mocks = vi.hoisted(() => ({
   songToTrack: vi.fn(),
 }));
 
-vi.mock('../../api/subsonicLibrary', () => ({
+vi.mock('@/lib/api/subsonicLibrary', () => ({
   getAlbum: mocks.getAlbum,
   getSong: mocks.getSong,
 }));
@@ -31,7 +31,7 @@ vi.mock('@/features/artist', () => ({
   getArtist: mocks.getArtist,
 }));
 
-vi.mock('../../api/subsonicEntityWithCredentials', () => ({
+vi.mock('@/lib/api/subsonicEntityWithCredentials', () => ({
   getAlbumWithCredentials: mocks.getAlbumWithCredentials,
   getArtistWithCredentials: mocks.getArtistWithCredentials,
   getSongWithCredentials: mocks.getSongWithCredentials,