Enhance CSRF protection and user management features

- Introduce CSRF token generation and management improvements, ensuring tokens are consistently retrieved from cookies or request state.
- Update user creation logic to handle validation errors more gracefully, providing user feedback for invalid input and existing email addresses.
- Revise user management UI to display success and error messages, improving user experience during user creation.
- Refactor admin user access checks to include additional role validation.

app/services/auth.py

@@ -36,7 +36,7 @@ def get_current_user(request: Request, db: Session = Depends(get_db)) -> User:
 
 
 def get_admin_user(current_user: User = Depends(get_current_user)) -> User:
-    if current_user.role != "admin":
+    if current_user.role != "admin" and not current_user.is_admin:
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Admin-Rechte erforderlich.")
     return current_user