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/unit/device_event_test.dart

@@ -0,0 +1,43 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/model/device_event.dart';
+
+import '../helpers/fixtures.dart';
+
+void main() {
+  test('DeviceEvent.fromJson maps all fields', () {
+    final event = DeviceEvent.fromJson(
+      deviceEventJson(
+        id: 42,
+        macAddress: '11:22:33:44:55:66',
+        createdOn: '2026-05-20T10:30:00Z',
+        eventType: 'changed',
+        ipv4Address: '10.0.0.9',
+        vendor: 'Initech',
+        scanner: 'Ssdp',
+      ),
+    );
+
+    expect(event.id, 42);
+    expect(event.macAddress, '11:22:33:44:55:66');
+    expect(event.createdOn, DateTime.parse('2026-05-20T10:30:00Z'));
+    expect(event.eventType, 'changed');
+    expect(event.ipv4Address, '10.0.0.9');
+    expect(event.vendor, 'Initech');
+    expect(event.scanner, 'Ssdp');
+  });
+
+  test('scannerLabel humanises known scanner names', () {
+    expect(_label('Arp'), 'ARP');
+    expect(_label('Mdns'), 'mDNS');
+    expect(_label('Ssdp'), 'SSDP/UPnP');
+    expect(_label('Dhcp'), 'DHCP');
+    expect(_label('Snmp'), 'SNMP');
+  });
+
+  test('scannerLabel passes through unknown scanner names', () {
+    expect(_label('Custom'), 'Custom');
+  });
+}
+
+String _label(String scanner) =>
+    DeviceEvent.fromJson(deviceEventJson(scanner: scanner)).scannerLabel;