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/login.php

@@ -33,12 +33,12 @@ $pageTitle = 'Connexion';
                 <div class="admin-alert admin-alert--error">⚠ <?= htmlspecialchars($error) ?></div>
             <?php endif; ?>
             <form method="post" action="/admin/login.php" class="admin-form">
-                <div class="admin-form-row" style="grid-template-columns:1fr;border:none;padding:.4rem 0;">
-                    <label class="admin-label" style="font-size:.82rem;color:var(--admin-text-muted);" for="password">Mot de passe</label>
+                <div class="admin-form-row">
+                    <label class="admin-label" for="password">Mot de passe</label>
                     <input class="admin-input" type="password" id="password" name="password" required autofocus>
                 </div>
-                <div class="admin-submit-wrap" style="margin-top:1rem;padding-top:.5rem;">
-                    <button type="submit" class="admin-btn" style="width:100%;">Se connecter</button>
+                <div class="admin-submit-wrap">
+                    <button type="submit" class="admin-btn">Se connecter</button>
                 </div>
             </form>
         </div>