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/data/services/iperf3_service.dart

@@ -0,0 +1,47 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:iperf3gui/domain/models/iperf3_result.dart';
+import 'package:iperf3gui/domain/models/test_configuration.dart';
+
+/// Service that runs iperf3 via [Process.spawn] and captures output.
+class Iperf3Service {
+  final String _binaryPath;
+
+  // ignore: prefer_initializing_formals
+  Iperf3Service({required String binaryPath}) : _binaryPath = binaryPath;
+
+  /// Run iperf3 with the given configuration.
+  ///
+  /// Returns an [Iperf3Result] with stdout/stderr and exit code.
+  Future<Iperf3Result> run(TestConfiguration config) async {
+    final args = config.toCliArgs();
+
+    final process = await Process.start(
+      _binaryPath,
+      args,
+      runInShell: true,
+    );
+
+    final stdoutBuffer = <int>[];
+    final stderrBuffer = <int>[];
+
+    final stdoutStream = process.stdout.transform(utf8.decoder).listen(
+      (data) => stdoutBuffer.addAll(utf8.encode(data)),
+    );
+    final stderrStream = process.stderr.transform(utf8.decoder).listen(
+      (data) => stderrBuffer.addAll(utf8.encode(data)),
+    );
+
+    await process.exitCode;
+    await stdoutStream.cancel();
+    await stderrStream.cancel();
+
+    final rawOutput = utf8.decode(stdoutBuffer) + utf8.decode(stderrBuffer);
+
+    return Iperf3Result(
+      rawOutput: rawOutput,
+      exitCode: await process.exitCode,
+    );
+  }
+}