Add system-preference-based theme support

[tofarr]

This PR adds support for automatically switching between light and dark themes based on the user's system preferences.

Changes:

• Updated NextUI configuration to support both light and dark themes with appropriate colors
• Created a new ThemeContext that:
  • Automatically detects system's color scheme preference
  • Persists theme preference in localStorage
  • Listens for system theme changes
  • Provides a way to manually switch themes
• Updated root layout to wrap the application with ThemeProvider

The theme will now:

• Use system's color scheme preference by default
• Remember user's last selected theme
• Update in real-time if system's theme preference changes
• Apply appropriate theme classes to HTML root element

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:96cad63-nikolaik   --name openhands-app-96cad63   docker.all-hands.dev/all-hands-ai/openhands:96cad63