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/RegisterPage.client.js

@@ -97,7 +97,7 @@ export default function RegisterPage({ onSubmit, onNavigate, currentUser = null
   }
 
   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="Créer un compte" description="Inscrivez-vous pour commencer." />
 
       {currentUser && (