fix: improve header and footer visual distinction

[adarshofficial560-ux]

## Description
The header and footer didn't really stand out from the rest of the page before they just blended in with the content, making the layout feel flat and unstructured.

So I made a few small but noticeable changes:

1. Added a bottom border to the navbar so it clearly separates from the page content below it.
2. Gave the footer a distinct background so it feels grounded and separate from the main section above it.
3. Made sure everything works properly in both light and dark mode; the footer uses a light gray in light mode and a darker tone in dark mode.
4. Many more ...hover effects and all.

Related Issue

close [#1369]

Type of Contribution

• Bug fix
• New feature
• Documentation update
• Refactor
• GSSoC contribution

Participant Info

• GitHub username: adarshofficial560-ux
• Contribution level (Beginner/Intermediate/Advanced): Beginner

Screen Recording

https://www.loom.com/share/07c87319dd0841abadbaa84836c32920

Recording / Loom link: ## Checklist

• [x ] I have read the contribution guidelines
• [ x] My changes follow the project structure
• [x ] I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• [x ] New interactive elements have aria-label / accessible names
• No console.log statements left in
• [

x] This PR is related to a valid issue - [x ] **Screen recording attached above** (required for UI/feature/design changes)

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📖 CONTRIBUTING.md

[github-actions]

👋 Thanks for your PR, @adarshofficial560-ux!

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