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/settings/hooks/useUserMgmtActions.ts

@@ -7,7 +7,7 @@ import {
   ndUpdateUser,
   type NdLibrary,
   type NdUser,
-} from '@/api/navidromeAdmin';
+} from '@/lib/api/navidromeAdmin';
 import { showToast } from '@/utils/ui/toast';
 import {
   copyTextToClipboard,