feat: Dark Mode

[JustinhSE]

Overview

The dark mode toggle feature (moon/sun icons) should be available in the navbar. We need to add this toggle, placing it clearly in the navbar left of the login/sign-up icon.

To Do

• Add dark mode toggle functionality (moon/sun alternating symbols). Use lucide-react icons.
• Position dark mode toggle clearly in navbar, left of login/sign-up icon.
• Ensure toggle correctly switches between dark and light modes.