style(ui): navigation du header centrée

src/components/app-header.tsx

@@ -3,7 +3,7 @@ import Link from 'next/link'
 export function AppHeader() {
   return (
     <header className="border-b border-slate-200 bg-white">
-      <nav className="mx-auto flex max-w-5xl items-center gap-6 px-4 py-3">
+      <nav className="mx-auto flex max-w-5xl items-center justify-center gap-6 px-4 py-3">
         <Link href="/" className="text-lg font-bold text-slate-900">
           Flashcards
         </Link>