Improving 404-error page layout

Currently, the 404 page only indicates the error through the page title. To enhance user experience, the layout should include a clear, visible indicator that this is a 404-error page.

<img width="1021" height="427" alt="Image" src="https://github.com/user-attachments/assets/8d2b0dba-3091-4cfd-b2f3-9b3111608bf3" />