added 404 website themed page

[anushka11p]

Description

Adds a custom 404.html page served by the Express catch-all middleware for all unmatched routes. Previously, undefined routes (e.g. /xyz) returned a blank Express fallback with no way back to the app. This fix serves a themed 404 page consistent with the site's terminal/matrix aesthetic.

Linked Issue

Fixes #97

Changes Made

• Added catch-all app.use() handler at the end of server.js that serves frontend/404.html with HTTP status 404
• Created frontend/404.html with matrix rain background, green terminal palette, dynamic path injection via window.location.pathname, and navigation links back to /leaderboard and /registration

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• My code follows the project's coding style
• I have formatted my code locally using Prettier
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have updated documentation if required
• I have linked the relevant issue

Screenshots / Screen Recording

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[jagdish-15]

/format

[github-actions]

I have successfully run Prettier and pushed the formatting fixes to this PR.

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[jagdish-15]

I noticed the page still has responsiveness issues on smaller mobile screens, so I'd like to get that addressed before merging.

I've already pushed a small maintainer commit for some minor layout/styling refinements and also ran formatting on the PR, so please pull the latest changes from your branch before continuing.

Once the mobile responsiveness issues are resolved as well, this should be good to merge.