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/lib/utils/backend_reachability.dart

@@ -38,6 +38,16 @@ class BackendReachability extends ChangeNotifier {
     _prober = prober;
   }
 
+  /// Test-only hook to force a deterministic online state so polling widgets
+  /// load during tests (where connectivity_plus has no platform backing).
+  @visibleForTesting
+  void forceOnlineForTesting() {
+    _deviceHasNetwork = true;
+    _isBackendReachable = true;
+    _lastErrorMessage = null;
+    notifyListeners();
+  }
+
   void recordSuccess() {
     _lastSuccessAt = DateTime.now();
     _lastErrorMessage = null;