feat: 新增寄送驗證碼前 email 驗證

lib/RegisterPage.dart

@@ -4,6 +4,7 @@ import 'dart:async';
 import 'dart:math';
 import 'package:mailer/mailer.dart';
 import 'package:mailer/smtp_server.dart';
+import 'package:validators/validators.dart' as validator;
 
 void main() {
   runApp(MaterialApp(
@@ -100,6 +101,11 @@ class _RegisterPageState extends State<RegisterPage> {
         SnackBar(content: Text('請輸入電子信箱地址')),
       );
       return;
+    } else if (!validator.isEmail(email)) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text('請輸入正確的電子信箱')),
+      );
+      return;
     }
 
     _generatedCode = _generateVerificationCode();