feat: complete MDX migration, stabilize environment & verify via E2E tests

Former-commit-id: ec3e64156a2e182535cbdcf0d975cd54603a517d

components/layout/Header.tsx

@@ -95,10 +95,10 @@ export function Header({ navLinks }: HeaderProps) {
             <div className="h-4 w-px bg-white/20 mx-2" />
             <LanguageSwitcher />
             <Link
-              href={`/${currentLocale}/kontakt`}
+              href={`/${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"
             >
-              Kontakt
+              {currentLocale === 'de' ? 'Kontakt' : 'Contact'}
             </Link>
           </nav>
 
@@ -154,10 +154,10 @@ export function Header({ navLinks }: HeaderProps) {
               })}
               
               <Link 
-                href={`/${currentLocale}/kontakt`} 
+                href={`/${currentLocale}/${currentLocale === 'de' ? 'kontakt' : 'contact'}`} 
                 className="text-2xl font-heading font-bold text-primary border-b border-neutral-100 pb-4"
               >
-                Kontakt
+                {currentLocale === 'de' ? 'Kontakt' : 'Contact'}
               </Link>
               
               <div className="mt-8 pt-6">