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_binary_path_provider.dart

@@ -0,0 +1,55 @@
+import 'dart:io';
+
+import 'package:flutter/services.dart';
+import 'package:path_provider/path_provider.dart';
+
+/// Provides the path to the iperf3 binary on the device.
+///
+/// Call [initialize] in [main] (before [runApp]) to extract the
+/// bundled asset and populate the cache. After initialization,
+/// [path] returns a synchronous [String].
+class Iperf3BinaryPathProvider {
+  static String? _cachedPath;
+
+  /// Extract the bundled iperf3 binary to the app's cache directory.
+  ///
+  /// Call this once in [main] before [runApp].
+  static Future<void> initialize() async {
+    if (_cachedPath != null) return;
+
+    final cacheDir = await getApplicationCacheDirectory();
+    final binaryDir = Directory('${cacheDir.path}/iperf3');
+    if (!await binaryDir.exists()) {
+      await binaryDir.create(recursive: true);
+    }
+
+    final abi = await _detectAbi();
+    final assetPath = 'assets/iperf3/$abi/iperf3';
+
+    final bytes = await rootBundle.load(assetPath);
+    final file = File('${binaryDir.path}/iperf3');
+    await file.writeAsBytes(bytes.buffer.asUint8List());
+
+    // chmod +x the binary
+    await Process.run('chmod', ['+x', file.path]);
+
+    _cachedPath = file.path;
+  }
+
+  /// The resolved path to the iperf3 binary.
+  ///
+  /// Must call [initialize] first, or this will return a placeholder.
+  static String get path {
+    return _cachedPath ?? '/data/local/tmp/iperf3';
+  }
+
+  static Future<String> _detectAbi() async {
+    final result = await Process.run(
+      'getprop',
+      ['ro.product.cpu.abi'],
+    );
+    final abi = result.stdout.toString().trim();
+    if (abi.contains('arm64')) return 'arm64-v8a';
+    return 'armeabi-v7a';
+  }
+}

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,
+    );
+  }
+}