Add Dark/Light Mode Theme Toggle

[kenny-io]

Add theme toggle functionality to improve user experience and accessibility. The app currently only supports a single theme.

Requirements:

• Add theme toggle button in the navigation bar
• Implement both dark and light color schemes using Tailwind
• Persist user theme preference in local storage
• Support system theme preference by default
• Ensure all components maintain proper contrast ratios in both themes

Technical Notes:

• Use Tailwind's dark mode configuration
• Implement theme context using React context API
• Component color schemes should be defined using CSS variables for maintainability
• Follow existing design system while adapting colors for both themes

[thuanvnnet]

Great idea. It is needed to improve user experience

[mzkrasner]

Initial take: #20 @kenny-io

[kenny-io]

thank you for your contribution @mzkrasner will take a look and feedback!