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/nl/settings.ts

@@ -332,6 +332,7 @@ export const settings = {
   themeStoreLoading: "Thema's laden…",
   themeStoreError: 'Kan de themawinkel niet laden. Controleer je verbinding.',
   themeStoreRetry: 'Opnieuw proberen',
+  themeStoreOfflineTitle: 'De Theme Store is offline',
   themeStoreEmpty: "Geen thema's komen overeen met je filters.",
   themeStoreActive: 'Actief',
   themeStoreEnlarge: 'Voorbeeld vergroten',