refactor: fix lint issues and wire up main screen

- Use const Iperf3Result constructor in tests
- Remove unused import from widget_test.dart
- Fix unnecessary underscore warnings in Consumer builders
- Use initializing formal for iperf3Service field
- Wire up main.dart to call Iperf3BinaryPathProvider.initialize()

lib/ui/features/test_run/test_run_view_model.dart

@@ -8,7 +8,7 @@ enum TestRunState { idle, running, completed, error }
 
 /// ViewModel for managing test run state.
 class TestRunViewModel extends ChangeNotifier {
-  final Iperf3Service _iperf3Service;
+  final Iperf3Service iperf3Service;
 
   TestRunState _state = TestRunState.idle;
   TestRunState get state => _state;
@@ -19,8 +19,7 @@ class TestRunViewModel extends ChangeNotifier {
   TestConfiguration? _currentConfig;
   TestConfiguration? get currentConfig => _currentConfig;
 
-  TestRunViewModel({required Iperf3Service iperf3Service})
-      : _iperf3Service = iperf3Service;
+  TestRunViewModel({required this.iperf3Service});
 
   /// Start a test with the given configuration.
   Future<void> runTest(TestConfiguration config) async {
@@ -36,7 +35,7 @@ class TestRunViewModel extends ChangeNotifier {
     notifyListeners();
 
     try {
-      final result = await _iperf3Service.run(config);
+      final result = await iperf3Service.run(config);
       _result = result;
       _state = result.exitCode == 0
           ? TestRunState.completed