fix(themes): offline banner for the Theme Store (#1013)

* fix(themes): show an offline banner when the Theme Store is unavailable

When the registry can't be fetched and no catalogue is cached, the store
now shows a clear offline banner (icon, message, retry) in place of the
list, and hides the search/filter toolbar — there is nothing to browse.
The cached-catalogue fallback with its offline indicator is unchanged, so
the store still works offline when a catalogue was previously cached.

* docs(themes): note the offline-banner PR in the Theme Store entry

Add PR #1013 to the still-unreleased Theme Store changelog and credits
entry, alongside the other follow-ups.

src/locales/zh/settings.ts

@@ -331,6 +331,7 @@ export const settings = {
   themeStoreLoading: '正在加载主题…',
   themeStoreError: '无法加载主题商店。请检查网络连接。',
   themeStoreRetry: '重试',
+  themeStoreOfflineTitle: '主题商店已离线',
   themeStoreEmpty: '没有符合筛选条件的主题。',
   themeStoreActive: '使用中',
   themeStoreEnlarge: '放大预览',