style: login button bg matches register banner (backdrop-blur-md)

frontend/src/components/Header.tsx

@@ -32,7 +32,7 @@ export default function Header() {
           <>
             <Link
               href="/auth/login"
-              className="rounded-lg bg-white/10 px-4 py-2 text-sm text-white hover:bg-white/20 transition"
+              className="rounded-lg bg-white/10 backdrop-blur-md px-4 py-2 text-sm text-white hover:bg-white/20 transition"
             >
               Войти
             </Link>