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/components/settings/ThemeStoreSection.test.tsx

@@ -143,4 +143,14 @@ describe('ThemeStoreSection — pagination & refresh', () => {
     await waitFor(() => expect(container.querySelector('.animate-spin')).toBeNull());
     expect(rows(container)).toHaveLength(12);
   });
+
+  it('shows an offline banner and hides the toolbar when the registry is unavailable', async () => {
+    fetchRegistryMock.mockRejectedValue(new Error('offline'));
+    renderWithProviders(<ThemeStoreSection />);
+
+    expect(await screen.findByText('The Theme Store is offline')).toBeInTheDocument();
+    // No catalogue to browse → the search/filter toolbar is not rendered.
+    expect(screen.queryByRole('searchbox')).not.toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Retry' })).toBeInTheDocument();
+  });
 });