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/app/Http/Controllers/Api/V1/PublicRegistrationDataController.php

@@ -100,7 +100,6 @@ final class PublicRegistrationDataController extends Controller
                         'normalized_options' => $field->normalized_options,
                         'tag_category' => $field->tag_category,
                         'is_required' => $field->is_required,
-                        'section' => $field->section,
                         'help_text' => $field->help_text,
                         'display_width' => $field->display_width->value,
                     ];