feat: HEADING field type for registration forms — replace section property with structural field

Replace the per-field `section` text property with a dedicated HEADING field type that
organizers add as a separate block for visual grouping. Also fixes duplicate heading bug
on portal radio fields, replaces cramped VBtnToggle with VSelect for field width, and
adds grouped field type dropdown with structure/input categories.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/database/factories/RegistrationFieldTemplateFactory.php

@@ -32,7 +32,6 @@ final class RegistrationFieldTemplateFactory extends Factory
             'is_filterable' => false,
             'is_portal_visible' => true,
             'is_admin_only' => false,
-            'section' => null,
             'help_text' => null,
             'sort_order' => fake()->numberBetween(0, 20),
             'display_width' => FieldDisplayWidth::FULL,
@@ -84,4 +83,15 @@ final class RegistrationFieldTemplateFactory extends Factory
             'display_width' => FieldDisplayWidth::FULL,
         ]);
     }
+
+    public function headingField(): static
+    {
+        return $this->state(fn () => [
+            'label' => 'Persoonlijke voorkeuren',
+            'slug' => 'persoonlijke-voorkeuren',
+            'field_type' => RegistrationFieldType::HEADING,
+            'help_text' => 'Vertel ons wat we over jou moeten weten',
+            'display_width' => FieldDisplayWidth::FULL,
+        ]);
+    }
 }