feat: add dark/light theme toggle with mobile support

[chethanamp22-tech]

### Closes: #385

Summary

Adds a dark/light theme toggle to the navbar so users can switch themes based on their preference in mobile.

Changes Made

• ThemeProvider.js — Context provider that manages theme state, persists to localStorage, and respects system prefers-color-scheme on first visit
• Navbar.js — Added toggle button (🌙/☀️) visible in navbar on both desktop and mobile
• Navbar.css — Dark mode styles via html[data-theme="dark"] selectors, toggle visible in top bar on mobile
• index.css — CSS variables (--bg, --text, --card, --border, etc.) for both light and dark themes

Checklist

• Toggle visible in navbar on desktop
• Toggle visible in navbar on mobile
• All pages update on switch
• Preference saved across reloads via localStorage
• Respects system prefers-color-scheme on first visit

Screenshots

Before:

After:

[vercel]

@chethanamp22-tech is attempting to deploy a commit to the Lovely Mahour's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Stewartsson]

@lovelymahor I am a GSSoC contributor and i wish to work in this issue and complete it

[chethanamp22-tech]

@lovelymahor I am a GSSoC contributor and i wish to work in this issue and complete it

I have completed this you can search for another issue,thanks!