Make site section vocabulary configurable and simplify settings form

Add a vocabulary selector to the settings form so the site section
vocabulary is no longer hardcoded. Remove node bundle checkboxes
(section_page/content_page are always included), keep user and group
as configurable parent targets. All hardcoded 'site_sections'
references in PHP replaced with dynamic config reads via helper.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Plugin/ParentEntityHandler/TaxonomyTermHandler.php

@@ -20,7 +20,7 @@ use Drupal\taxonomy\TermInterface;
   entity_type_id: 'taxonomy_term',
   clears_site_section: FALSE,
   sort_field: 'name',
-  bundle_restrictions: ['site_sections'],
+  bundle_restrictions: [],
   weight: 0,
 )]
 class TaxonomyTermHandler extends ParentEntityHandlerBase {
@@ -61,6 +61,19 @@ class TaxonomyTermHandler extends ParentEntityHandlerBase {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function handlesEntity(EntityInterface $entity): bool {
+    if ($entity->getEntityTypeId() !== 'taxonomy_term') {
+      return FALSE;
+    }
+
+    $vocabulary = \Drupal::config('structural_pages.settings')
+      ->get('site_section_vocabulary') ?? 'site_sections';
+    return $entity->bundle() === $vocabulary;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -69,8 +82,7 @@ class TaxonomyTermHandler extends ParentEntityHandlerBase {
       return NULL;
     }
 
-    // Only site_sections terms can be site sections.
-    if ($entity->bundle() !== 'site_sections') {
+    if (!$this->handlesEntity($entity)) {
       return NULL;
     }