[Frontend] Dark/light mode theme toggle with persistent preference

[Gbangbolaoluwagbemiga]

Overview

`next-themes` is already installed but there's no toggle in the UI. Users on light OS themes are stuck with the dark dApp.

What needs to be done

1. Add sun/moon icon toggle button to the navbar (Lucide icons already installed)
2. Wire to `next-themes`: `const { theme, setTheme } = useTheme()`
3. Verify CSS variables defined for both `:root` (light) and `.dark` in `src/index.css`
4. Confirm `ThemeProvider` wraps app in `src/main.tsx`
5. Audit all pages render correctly in light mode

Acceptance criteria

• Toggle visible in navbar
• Preference persists after page refresh
• All pages readable in both themes
• Respects `prefers-color-scheme` on first visit (no hydration flash)

Good first issue: Touches layout/theming only, no contract logic.

[rajveersingh-07]

Hi Gbangbolaoluwagbemiga ,I’d like to work on this issue under GSSoC. Could you please assign it to me?