Add light and dark mode support

[tac0turtle]

Summary

Add support for light and dark mode themes to the application.

Details

• Implement a theme toggle (light/dark mode)
• Respect user's system preference by default
• Persist theme selection across sessions

Acceptance Criteria

• Light mode theme implemented
• Dark mode theme implemented
• Theme toggle accessible in the UI
• System preference detection (prefers-color-scheme)
• Theme persisted in local storage