style(auth): improve auth page layout for mobile viewports

- use `min-h-dvh`, `flex-col`, and top-aligned justify on small screens in AuthPage
- add `mx-auto` to all auth page cards for consistent centering

src/features/auth/pages/LoginPage.client.js

@@ -67,7 +67,7 @@ export default function LoginPage({ onSubmit, onNavigate, onSetSessionCookie, re
   };
 
   return (
-    <Card variant="default" padding="md" spacing="none" className="w-full max-w-md">
+    <Card variant="default" padding="md" spacing="none" className="w-full max-w-md mx-auto">
       <AuthPageHeader title="Connexion" description="Veuillez vous connecter pour continuer." />
 
       {currentUser && (