refactor: enforce global Button component usage across all files (Header, Forms, Grids, MDX)

Former-commit-id: aa55620667a28e5c00ab2603da6ea6dc1e1c5ecf

components/layout/Header.tsx

@@ -6,6 +6,7 @@ import Image from 'next/image';
 import { usePathname } from 'next/navigation';
 import { motion, AnimatePresence } from 'framer-motion';
 import { LanguageSwitcher } from './LanguageSwitcher';
+import { Button } from '@/components/ui/Button';
 
 export interface NavLink {
   label: string;
@@ -108,12 +109,13 @@ export function Header({ navLinks }: HeaderProps) {
             })}
             <div className={`h-4 w-px mx-2 transition-colors duration-300 ${isSolidMode ? 'bg-neutral-300' : 'bg-white/20'}`} />
             <LanguageSwitcher isSolidMode={isSolidMode} />
-            <Link
+            <Button
               href={`/${currentLocale}/${currentLocale === 'de' ? 'kontakt' : 'contact'}`}
-              className="rounded-xl bg-primary hover:bg-primary-dark text-neutral-light px-8 py-3 font-bold text-sm uppercase tracking-wider transition-all shadow-xl hover:shadow-primary/20 hover:-translate-y-0.5 active:translate-y-0"
+              variant="primary"
+              className="px-8"
             >
               {currentLocale === 'de' ? 'Kontakt' : 'Contact'}
-            </Link>
+            </Button>
           </nav>
 
           {/* Mobile Nav Button */}