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/device_summary_card_test.dart

@@ -0,0 +1,70 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/widgets/device_summary_card.dart';
+import 'package:http_mock_adapter/http_mock_adapter.dart';
+
+import '../helpers/backend_test_harness.dart';
+import '../helpers/fixtures.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  late DioAdapter adapter;
+
+  setUp(() async {
+    adapter = await setUpBackendForTest();
+  });
+
+  testWidgets('shows a loading spinner before the summary arrives',
+      (tester) async {
+    adapter.onGet(
+      '/devices/summary',
+      (server) => server.reply(200, deviceSummaryJson()),
+    );
+
+    await pumpScreen(tester, const DeviceSummaryCard());
+
+    expect(find.byType(CircularProgressIndicator), findsOneWidget);
+
+    // Let the in-flight request resolve so no Dio timer is left pending.
+    await pumpUntilFound(tester, find.byType(Divider));
+    await tearDownTree(tester);
+  });
+
+  testWidgets('renders the summary numbers after loading', (tester) async {
+    adapter.onGet(
+      '/devices/summary',
+      (server) => server.reply(
+        200,
+        deviceSummaryJson(totalRegistered: 12, seenLastDayRegistered: 3),
+      ),
+    );
+
+    await pumpScreen(tester, const DeviceSummaryCard());
+    await pumpUntilFound(tester, find.text('12'));
+
+    expect(find.text('12'), findsOneWidget);
+    expect(find.text('3'), findsWidgets);
+
+    await tearDownTree(tester);
+  });
+
+  testWidgets('shows the error message when the request fails', (tester) async {
+    adapter.onGet(
+      '/devices/summary',
+      (server) => server.reply(500, {'error': 'boom'}),
+    );
+
+    await pumpScreen(tester, const DeviceSummaryCard());
+    await pumpUntilFound(
+      tester,
+      find.textContaining('Backend error (status 500)'),
+    );
+
+    expect(
+      find.textContaining('Backend error (status 500)'),
+      findsOneWidget,
+    );
+
+    await tearDownTree(tester);
+  });
+}