chore: Linter fixes for USB serial transport

lib/services/serial/usb_serial_transport.dart

@@ -1,5 +1,4 @@
 import 'dart:async';
-import 'dart:typed_data';
 import 'package:flutter/foundation.dart';
 import 'package:usb_serial/usb_serial.dart';
 import 'package:meshcore_client/meshcore_client.dart';

lib/services/serial/web_serial_transport.dart

@@ -1,6 +1,4 @@
 import 'dart:async';
-import 'dart:typed_data';
-import 'package:flutter/foundation.dart';
 import 'package:meshcore_client/meshcore_client.dart';
 
 // Web Serial API is only available on web platform.

lib/widgets/connection_dialog.dart

@@ -758,6 +758,7 @@ class _UsbDeviceListState extends State<_UsbDeviceList> {
         widget.onConnected();
       } else {
         await port.close();
+        if (!mounted) return;
         setState(() => _isConnecting = false);
         ScaffoldMessenger.of(context).showSnackBar(
           const SnackBar(content: Text('Failed to connect via USB')),