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()

test/ui/features/test_run/test_run_view_model_test.dart

@@ -36,7 +36,7 @@ void main() {
       final mock = _MockIperf3Service(
         onRun: (config) async {
           expect(config.server, equals('192.168.1.1'));
-          return Iperf3Result(rawOutput: 'done', exitCode: 0);
+          return const Iperf3Result(rawOutput: 'done', exitCode: 0);
         },
       );
       final vm = TestRunViewModel(iperf3Service: mock);
@@ -56,6 +56,6 @@ class _MockIperf3Service implements Iperf3Service {
   @override
   Future<Iperf3Result> run(TestConfiguration config) async {
     if (onRun != null) return onRun!(config);
-    return Iperf3Result(rawOutput: '', exitCode: 0);
+    return const Iperf3Result(rawOutput: '', exitCode: 0);
   }
 }