Add unit tests for LoginButton component with comprehensive coverage

backend/api/user_api.py

@@ -5,8 +5,7 @@ from db.db import users_db
 import jwt
 import random
 import string
-import smtplib
-from backend.utils.email_instance import email_sender
+import utils.email_sender as email_sender
 from datetime import datetime, timedelta
 from api.utils import get_validated_user_id