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/ConfirmEmailPage.client.js

@@ -80,7 +80,7 @@ export default function ConfirmEmailPage({ onSubmit, onNavigate, email, token })
   console.log('ConfirmEmailPage render', { success, error, isLoading, hasVerified });
 
   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="Vérification de l'e-mail" description="Nous vérifions votre adresse e-mail..." />
 
       {isLoading && (