Remove the animated underline on header nav links

Nav links now use the same plain colour transition as the mobile menu
and footer links.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jWbNNJLksC1QG8z8845FX

components/shopify/header.tsx

@@ -103,7 +103,7 @@ const Header: React.FC<HeaderProps> = ({
               <Link
                 key={index}
                 href={link.url}
-                className="text-foreground relative after:absolute after:bottom-[-4px] after:left-1/2 after:-translate-x-1/2 after:h-px after:w-0 after:bg-foreground after:transition-[width] after:duration-300 hover:after:w-full"
+                className="text-foreground hover:text-muted-foreground transition-colors"
               >
                 {link.label}
               </Link>