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/features/waveform/components/WaveformSeek.test.tsx

@@ -8,7 +8,7 @@
  */
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
-vi.mock('@/api/subsonic', () => ({
+vi.mock('@/lib/api/subsonic', () => ({
   savePlayQueue: vi.fn(async () => undefined),
   getPlayQueue: vi.fn(async () => ({ songs: [], current: undefined, position: 0 })),
   buildStreamUrl: vi.fn((id: string) => `https://mock/stream/${id}`),