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/zh/connection.ts

@@ -11,6 +11,7 @@ export const connection = {
   offlineNoCacheBanner: '无服务器连接 — 无法访问 {{server}}',
   offlineLibraryTitle: '离线音乐库',
   offlineLibraryEmpty: '尚未缓存任何专辑。请联网，打开专辑并点击"设为离线可用"。',
+  offlineCachedOnServer: '来自 {{server}}',
   offlineAlbumCount_one: '{{n}} 张专辑',
   offlineAlbumCount_plural: '{{n}} 张专辑',
   retry: '重试',