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/pages-edit.php

@@ -88,7 +88,7 @@ JS;
         <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
         <input type="hidden" name="slug" value="<?= htmlspecialchars($slug) ?>">
 
-        <div class="admin-form-row" style="align-items:start;">
+        <div class="admin-form-row">
             <label class="admin-label" for="content">Contenu (Markdown) :</label>
             <div>
                 <textarea class="admin-textarea" id="content" name="content"
@@ -99,7 +99,7 @@ JS;
 
         <div class="admin-submit-wrap">
             <button type="submit" class="admin-btn">Enregistrer</button>
-            <a href="/admin/pages.php" class="admin-btn-secondary" style="margin-left:.75rem;">Annuler</a>
+            <a href="/admin/pages.php" class="admin-btn-secondary admin-cancel-link">Annuler</a>
         </div>
     </form>
 </main>