Add automated test suite for the Flutter frontend

Introduce a headless `flutter test` suite (83 tests) covering models,
utilities, every API endpoint, and the five screens, with shared helpers
and fixtures so new tests stay terse.

API endpoint methods are statically-dispatched extensions on the
BackendAPI singleton, so they cannot be mocked via `implements`. Mock at
the Dio HTTP-adapter layer (http_mock_adapter) instead, enabled by two
small @visibleForTesting seams: BackendAPI.dioForTesting swaps the
singleton's Dio, and BackendReachability.forceOnlineForTesting() forces a
deterministic online state so polling widgets load.

Add frontend/run_tests.sh and document it in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/about_test.dart

@@ -0,0 +1,25 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/about/about.dart';
+
+import '../helpers/backend_test_harness.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  setUp(() async {
+    await setUpBackendForTest();
+  });
+
+  testWidgets('renders the about content and release fallback', (tester) async {
+    await pumpScreen(tester, const About());
+    // PackageInfo.fromPlatform() has no plugin in tests, so the FutureBuilder
+    // falls back to the hard-coded release date.
+    await tester.pump();
+
+    expect(find.text('About OOTT'), findsOneWidget);
+    // Assert the fallback prefix only, not the release date literal (it changes
+    // every release).
+    expect(find.textContaining('Released'), findsOneWidget);
+    expect(find.text('Source code'), findsOneWidget);
+    expect(find.text('openssl'), findsOneWidget);
+  });
+}