Add 404 Page

The current behavior when going to a random URL is for the React app to load, but the router doesn't render any content template (i.e. nav bar with an empty pane).

https://econosim.org/randomURL

<img width="663" alt="Image" src="https://github.com/user-attachments/assets/6b0b132b-411d-47b2-83e5-552399d40627" />