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/status_screen_test.dart

@@ -0,0 +1,62 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/status/status_screen.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();
+  });
+
+  void stubAllScanners() {
+    adapter.onGet(
+      '/arp_scanner/status',
+      (server) => server.reply(
+        200,
+        intervalScannerJson(isRunning: true, runningForSeconds: 10),
+      ),
+    );
+    adapter.onGet(
+      '/mdns_scanner/status',
+      (server) => server.reply(
+        200,
+        listenerScannerJson(isListening: true, devicesSeen: 2),
+      ),
+    );
+    adapter.onGet(
+      '/ssdp_scanner/status',
+      (server) => server.reply(200, listenerScannerJson()),
+    );
+    adapter.onGet(
+      '/dhcp_scanner/status',
+      (server) => server.reply(200, listenerScannerJson()),
+    );
+    adapter.onGet(
+      '/snmp_scanner/status',
+      (server) => server.reply(200, intervalScannerJson()),
+    );
+  }
+
+  testWidgets('renders each scanner card with its resolved status',
+      (tester) async {
+    stubAllScanners();
+
+    await pumpScreen(tester, const StatusScreen());
+    await pumpUntilFound(tester, find.text('Running'));
+
+    expect(find.text('Status'), findsOneWidget);
+    expect(find.text('ARP Scanner'), findsOneWidget);
+    expect(find.text('mDNS Scanner'), findsOneWidget);
+    expect(find.text('Running'), findsWidgets);
+    expect(find.text('Listening'), findsWidgets);
+    expect(find.byType(CircularProgressIndicator), findsNothing);
+
+    await tearDownTree(tester);
+  });
+}