refactor: Extract GitHub icon to a separate component and replace Icon with Button

[O2sa]

Currently, we are using an Icon component exported from components/ui/button.tsx to render the GitHub link in both the ThemeToggle and AppFooter components. We want to clean this up by extracting the GitHub icon link into its own component and using the standard Button component.

Tasks

• Create a new component components/github-link.tsx that renders a Button containing the <FaGithub /> icon wrapped in an <a> tag.
• Replace the <Icon> usage with the new component in components/theme-toggle.tsx.
• Replace the <Icon> usage with the new component in components/app-footer.tsx.
• Remove the Icon component export from components/ui/button.tsx.