Improve 404 handling for browser routes

[moab01567]

Summary

This PR improves the browser experience for unknown routes by adding a simple custom 404 HTML page.

Before this change, unknown routes such as /jnfja returned the default FastAPI 404 Not Found JSON response in the browser. While this is technically correct, it gives a poor user experience for users who access an invalid URL through the browser.

The new custom 404 page explains that the requested page could not be found and gives the user a link back to the homepage. The response still uses status code 404, so the application does not hide the fact that the URL is invalid.

Why not redirect?

I considered redirecting unknown routes back to /, but chose not to do that because it could hide real routing errors. A custom 404 page keeps the error visible while making it easier for users to understand what happened.

Manual testing

• Verified that http://localhost:8004 still renders the homepage.
• Verified that http://localhost:8004/jnfja shows the custom 404 page.
• Verified that the unknown route is not redirected to /.
• Verified that the change improves the browser experience without changing the purpose of the error handling.