feat: Implement Dark/Light theme toggle on Landing Page

[AKASHDHARDUBEY]

Summary

This PR implements a Dark/Light theme toggle for the Landing Page to improve accessibility and user experience, addressing issue #675.

Changes

• Configuration: Enabled darkMode: 'class' in tailwind.config.js.
• State Management: Created ThemeContext to manage theme state and persist user preference in localStorage.
• Components:
  • Added ThemeToggle component with Sun/Moon icons.
  • Integrated the toggle into the Navbar (responsive for both desktop and mobile).
• Styling: Applied dark: variant classes to the LandingPage to ensure proper contrast and aesthetics in dark mode.

Verification

• The theme toggle works instantly when clicked.
• User preference is saved to localStorage and persists on reload.
• Defaults to system preference if no user setting is found.
• Verified UI consistency in both Light and Dark modes.

Issues

Fixes #675

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