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/helpers/backend_test_harness.dart

@@ -0,0 +1,67 @@
+import 'package:dio/dio.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/utils/backend_reachability.dart';
+import 'package:frontend/utils/oott_api.dart';
+import 'package:frontend/utils/pref_utils.dart';
+import 'package:http_mock_adapter/http_mock_adapter.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+const _connectivityMethodChannel = MethodChannel(
+  'dev.fluttercommunity.plus/connectivity',
+);
+const _connectivityEventChannel = EventChannel(
+  'dev.fluttercommunity.plus/connectivity_status',
+);
+
+bool _channelsStubbed = false;
+
+/// Neutralises the connectivity_plus platform channels so the
+/// [BackendReachability] singleton can be constructed in tests without the
+/// plugin throwing `MissingPluginException`. `check` reports a network, and
+/// the change stream stays open but emits nothing.
+void _stubConnectivityChannels() {
+  if (_channelsStubbed) return;
+  final messenger =
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
+  messenger.setMockMethodCallHandler(_connectivityMethodChannel, (call) async {
+    if (call.method == 'check') return <String>['wifi'];
+    return null;
+  });
+  messenger.setMockStreamHandler(
+    _connectivityEventChannel,
+    MockStreamHandler.inline(onListen: (arguments, events) {}),
+  );
+  _channelsStubbed = true;
+}
+
+/// Prepares the backend singletons for a test and returns a [DioAdapter] so the
+/// test can stub HTTP routes. Call from `setUp`.
+///
+/// This (1) seeds mock SharedPreferences, (2) stubs the connectivity channels,
+/// (3) forces [BackendReachability] online so polling widgets load, and
+/// (4) swaps the [BackendAPI] singleton's Dio for one backed by a mock adapter.
+///
+/// `prefs` are only honoured on the first call within a test isolate, because
+/// `PrefUtil` caches the SharedPreferences instance after its one-shot init.
+/// Use [PrefUtil.setValue] for per-test changes after that.
+Future<DioAdapter> setUpBackendForTest({
+  Map<String, Object> prefs = const {},
+}) async {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  _stubConnectivityChannels();
+
+  SharedPreferences.setMockInitialValues({
+    'base_url': 'http://test.local/api',
+    'api_key': '',
+    ...prefs,
+  });
+  await PrefUtil.init();
+
+  BackendReachability.instance.forceOnlineForTesting();
+
+  final dio = Dio(BaseOptions(baseUrl: 'http://test.local/api'));
+  final adapter = DioAdapter(dio: dio);
+  BackendAPI.instance.dioForTesting = dio;
+  return adapter;
+}