fix: use /system/bin/sh for chmod to fix iperf3 binary permission on Android

- The original chmod call used Process.run('chmod', ...) which fails on Android
  because chmod is not available in the app sandbox PATH
- /system/bin/sh is always present on Android and can execute chmod on the
  extracted binary, setting executable permissions before the first run

lib/data/services/iperf3_binary_path_provider.dart

@@ -30,8 +30,11 @@ class Iperf3BinaryPathProvider {
     final file = File('${binaryDir.path}/iperf3');
     await file.writeAsBytes(bytes.buffer.asUint8List());
 
-    // chmod +x the binary
-    await Process.run('chmod', ['+x', file.path]);
+    // chmod +x the binary — /system/bin/sh is always present on Android
+    await Process.run(
+      '/system/bin/sh',
+      ['-c', 'chmod +x "\$1"', '_', file.path],
+    );
 
     _cachedPath = file.path;
   }