toc : add h4 support with nested structure and fix anchors in non-block mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

site/snippets/toc.php

@@ -2,7 +2,16 @@
     <div class="light toc_label">table des matières</div>
     <ul>
         <?php foreach ($page->tocItems() as $item): ?>
-            <li><a href="#<?= $item['slug'] ?>"><?= $item['title'] ?></a></li>
+            <li>
+                <a href="#<?= $item['slug'] ?>"><?= $item['title'] ?></a>
+                <?php if (!empty($item['children'])): ?>
+                    <ul>
+                        <?php foreach ($item['children'] as $child): ?>
+                            <li><a href="#<?= $child['slug'] ?>"><?= $child['title'] ?></a></li>
+                        <?php endforeach ?>
+                    </ul>
+                <?php endif ?>
+            </li>
         <?php endforeach ?>
     </ul>
 </nav>