feat(email): add dynamic subject line for OTP emails

- Use template data for OTP code as the email subject
- Improve email personalization and user experience

Author: fulleni

lib/src/email_sendgrid.dart

@@ -50,6 +50,7 @@ class EmailSendGrid implements EmailClient {
         },
       ],
       'from': {'email': senderEmail},
+      'subject': templateData['otp_code'],
       'template_id': templateId,
     };