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/RegisterPage.client.js

@@ -2,6 +2,7 @@
 
 import { useState, useEffect } from 'react';
 import { Card, Input, Button, PasswordStrengthIndicator } from '@zen/core/shared/components';
+import AuthPageHeader from '../components/AuthPageHeader.js';
 
 export default function RegisterPage({ onSubmit, onNavigate, currentUser = null }) {
   const [error, setError] = useState('');
@@ -97,14 +98,7 @@ export default function RegisterPage({ onSubmit, onNavigate, currentUser = null
 
   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">
-          Créer un compte
-        </h1>
-        <p className="text-sm text-neutral-600 dark:text-neutral-400">
-          Inscrivez-vous pour commencer.
-        </p>
-      </div>
+      <AuthPageHeader title="Créer un compte" description="Inscrivez-vous pour commencer." />
 
       {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">