refactor(auth): extract shared page header into AuthPageHeader component

- add AuthPageHeader component to centralize title/description markup
- replace inline header divs in LoginPage, RegisterPage, LogoutPage, ForgotPasswordPage, ResetPasswordPage, and ConfirmEmailPage with AuthPageHeader

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

@@ -3,6 +3,7 @@
 import { useState, useEffect } from 'react';
 import { useRouter } from 'next/navigation';
 import { Card, Input, Button } from '@zen/core/shared/components';
+import AuthPageHeader from '../components/AuthPageHeader.js';
 
 export default function LoginPage({ onSubmit, onNavigate, onSetSessionCookie, redirectAfterLogin = '/', currentUser = null }) {
   const [error, setError] = useState('');
@@ -67,14 +68,7 @@ export default function LoginPage({ onSubmit, onNavigate, onSetSessionCookie, re
 
   return (
     <Card variant="default" padding="md" spacing="none" className="w-full max-w-md">
-      <div className="text-center mb-6">
-        <h1 className="text-2xl font-bold text-neutral-900 dark:text-white mb-2">
-          Connexion
-        </h1>
-        <p className="text-sm text-neutral-600 dark:text-neutral-400">
-          Veuillez vous connecter pour continuer.
-        </p>
-      </div>
+      <AuthPageHeader title="Connexion" description="Veuillez vous connecter pour continuer." />
 
       {currentUser && (
         <div className="mb-4 p-3 bg-blue-50 border border-blue-200 rounded-lg dark:bg-blue-500/10 dark:border-blue-500/20">