feat(offline): Offline Library shows cached albums from all servers (#719)

* feat(offline): show cached albums from all servers in Offline Library

List every offline album regardless of active server; load cover art per
source server and switch before play/enqueue. Sidebar, mobile nav, and
disconnect auto-nav use any cached content; multi-server cards show a label.

* docs(changelog): link offline library PR #719

* chore(pr-719): address review — CHANGELOG in Added, helper tests

Move release note to ## Added per team changelog policy; cover
offlineAlbumCoverArt and ensureServerForOfflineAlbum in unit tests.

src/locales/ro/connection.ts

@@ -11,6 +11,7 @@ export const connection = {
   offlineNoCacheBanner: 'Nicio conexiune la server — nu s-a putut ajunge la {{server}}',
   offlineLibraryTitle: 'Librărie offline',
   offlineLibraryEmpty: 'Niciun album adăugat în cache. Conectează-te, deschide un album și apasă "Fă disponibil offline".',
+  offlineCachedOnServer: 'Pe {{server}}',
   offlineAlbumCount: '{{n}} album',
   offlineAlbumCount_plural: '{{n}} albume',
   offlineFilterAll: 'Toate',