clear toast on tap

this adds a generator showDismissibleSnackBar which by default allows
tapping to clear snack bar toasts. all SnackBar properties are still
available and the

all callers should now use showDismissibleSnackBar() instead of calling
ScaffoldMessenger.of(context).showSnackBar(SnackBar())

lib/screens/usb_screen.dart

@@ -10,6 +10,7 @@ import '../utils/app_logger.dart';
 import '../utils/platform_info.dart';
 import '../utils/usb_port_labels.dart';
 import '../widgets/adaptive_app_bar_title.dart';
+import '../helpers/snack_bar_builder.dart';
 import 'contacts_screen.dart';
 import 'scanner_screen.dart';
 import 'tcp_screen.dart';
@@ -383,11 +384,10 @@ class _UsbScreenState extends State<UsbScreen> {
 
   void _showError(Object error) {
     if (!mounted) return;
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(
-        content: Text(_friendlyErrorMessage(error)),
-        backgroundColor: Colors.red,
-      ),
+    showDismissibleSnackBar(
+      context,
+      content: Text(_friendlyErrorMessage(error)),
+      backgroundColor: Colors.red,
     );
   }