fix: harden metadata loading failures

Author: gildesmarais

frontend/src/__tests__/App.contract.test.tsx

@@ -109,4 +109,17 @@ describe('App contract', () => {
     expect(slashlessMetadataRequests).toBeGreaterThanOrEqual(1);
     expect(trailingSlashMetadataRequests).toBe(0);
   });
+
+  it('shows the metadata unavailable notice when /api/v1 responds with non-JSON content', async () => {
+    server.use(
+      http.get('/api/v1', () => HttpResponse.text('not-json', { status: 502 })),
+      http.get('/api/v1/', () => HttpResponse.text('', { status: 404 }))
+    );
+
+    render(<App />);
+
+    await screen.findByText('Instance metadata unavailable');
+
+    expect(screen.getByText('Invalid response format from API metadata')).toBeInTheDocument();
+  });
 });

frontend/src/hooks/useApiMetadata.ts

@@ -7,6 +7,11 @@ interface ApiMetadataState {
   error: string | null;
 }
 
+interface ApiMetadataPayload {
+  success?: boolean;
+  data?: unknown;
+}
+
 export function useApiMetadata() {
   const [state, setState] = useState<ApiMetadataState>({
     metadata: null,
@@ -25,7 +30,7 @@ export function useApiMetadata() {
           signal: controller.signal,
           headers: { Accept: 'application/json' },
         });
-        const payload = (await response.json()) as { success?: boolean; data?: unknown };
+        const payload = await parseMetadataPayload(response);
         const metadata = payload.data as ApiMetadataRecord | undefined;
 
         if (!response.ok || !payload.success || !metadata?.instance) {
@@ -54,3 +59,14 @@ export function useApiMetadata() {
 
   return state;
 }
+
+async function parseMetadataPayload(response: Response): Promise<ApiMetadataPayload> {
+  const body = await response.text();
+  if (!body.trim()) return {};
+
+  try {
+    return JSON.parse(body) as ApiMetadataPayload;
+  } catch {
+    return {};
+  }
+}