✨ feat: improve user management and update styles for better UX

utils/user_manager.py

@@ -55,7 +55,7 @@ def create_user(username, email, password, is_admin=False):
             user = User(
                 username=username,
                 email=email,
-                is_admin=is_admin,
+                role='admin' if is_admin else 'user',
                 created_at=datetime.utcnow()
             )
             user.set_password(password)