add snackbar method for errors

test (ping) server before saving source
display error message when saving source

lib/app/snackbars.dart

@@ -0,0 +1,14 @@
+import 'package:flutter/material.dart';
+
+void showErrorSnackbar(BuildContext context, String message) {
+  final colors = Theme.of(context).colorScheme;
+
+  ScaffoldMessenger.of(context).showSnackBar(SnackBar(
+    content: Text(message, style: TextStyle(color: colors.onErrorContainer)),
+    backgroundColor: colors.errorContainer,
+    showCloseIcon: true,
+    closeIconColor: colors.onErrorContainer,
+    behavior: SnackBarBehavior.floating,
+    duration: const Duration(seconds: 10),
+  ));
+}