fix: replace default Firebase 404 page with M-Lab branded version

[Aaradhya-07]

Summary

The current 404.html is the default Firebase CLI template. It references Firebase internals ("generated by the Firebase Command-Line Interface") and has no M-Lab branding, which is confusing for users who land on a broken URL.

Changes made

Replaced app/404.html with a clean, branded page that matches the site's dark theme
Added the M-Lab favicon
Added a "Go to Speed Test" button linking back to /
Added proper lang attribute and page title

Why this change

Users hitting a non-existent URL should see a helpful, branded page — not a Firebase boilerplate that tells them to edit config files.

[robertodauria]

Thanks for the contribution. Closing this as #83 already addresses it. We aim to merge #83 next week, which will give contributors a stable, modern base to work from. We'd encourage you to review it and open any future PRs on top of that.