Phase 1: Replace Parsedown with league/commonmark (4 call sites)

app/templates/partials/form/form-help-block.php

@@ -7,7 +7,6 @@
  *   string $helpKey     — block key, used as element id.
  *
  * Outputs nothing when $helpContent is empty or whitespace-only.
- * Parsedown must already be autoloaded (it is, via bootstrap → APP_ROOT/src/).
  */
 
 $helpContent = trim($helpContent ?? '');
@@ -15,13 +14,11 @@ if ($helpContent === '') {
     return;
 }
 
-require_once APP_ROOT . '/src/Parsedown.php';
-$pd   = new Parsedown();
-$pd->setSafeMode(true);
-$html = $pd->text($helpContent);
+$converter = new League\CommonMark\CommonMarkConverter(['html_input' => 'strip']);
+$html = $converter->convert($helpContent)->getContent();
 ?>
 <div class="student-help-block" id="help-<?= htmlspecialchars($helpKey ?? 'unknown') ?>">
     <?= $html ?>
 </div>
 <?php
-unset($helpContent, $pd, $html);
+unset($helpContent, $converter, $html);