Remove inline styles from admin templates; extract to admin.css utility classes

- login.php: removed style= on .admin-form-row and .admin-label (already covered
  by .admin-login-box scoped rules); extracted submit-wrap spacing and full-width
  button to .admin-login-box .admin-submit-wrap and .admin-login-box .admin-btn
- account.php: style="margin-top:3rem" on danger-zone heading moved to
  .admin-section-title--danger modifier; <span style="color:..."> replaced with
  <small> element styled via .admin-danger-zone__description small
- add.php / edit.php / pages-edit.php: all style="align-items:start" removed from
  .admin-form-row (redundant — already the CSS default at line 116 of admin.css);
  banner preview inline styles extracted to .admin-banner-preview / .admin-banner-preview img;
  add-jury button margin extracted to .admin-add-jury-btn; cancel links use .admin-cancel-link

Zero inline style= attributes remain in login, account, add, edit, pages-edit.

public/admin/account.php

@@ -99,14 +99,14 @@ if (empty($_SESSION['csrf_token'])) {
 
     <?php if ($hasPassword): ?>
     <!-- Danger zone: remove password -->
-    <h2 class="admin-section-title admin-section-title--danger" style="margin-top:3rem;">Zone de danger</h2>
+    <h2 class="admin-section-title admin-section-title--danger">Zone de danger</h2>
     <div class="admin-danger-zone">
         <div class="admin-danger-zone__description">
             <strong>Supprimer la configuration du mot de passe PHP</strong><br>
-            <span style="color:var(--admin-text-muted);font-size:.9rem;">
+            <small>
                 Supprime <code>config/admin_credentials.php</code>. L'accès admin
                 dépendra uniquement de l'authentification nginx Basic Auth si elle est configurée.
-            </span>
+            </small>
         </div>
         <form method="post" action="/admin/actions/account.php"
               onsubmit="return confirm('Supprimer le fichier de mot de passe PHP ? L\'accès admin ne sera protégé que par nginx Basic Auth.')">