refactor: replace lucide-react with Radix icons and add GitHub link

Replace lucide-react dependency with Radix UI's native icon system.
Increase theme toggle icon size from 18px to 22px for better visibility.
Add GitHub repository link with icon button in the navigation header.

src/components/ThemeToggle.tsx

@@ -31,9 +31,9 @@ export const ThemeToggle = () => {
 			title={`Switch to ${theme === "light" ? "dark" : "light"} mode`}
 		>
 			{theme === "light" ? (
-				<MoonIcon width="18" height="18" />
+				<MoonIcon width="22" height="22" />
 			) : (
-				<SunIcon width="18" height="18" />
+				<SunIcon width="22" height="22" />
 			)}
 		</IconButton>
 	);