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/Requests/Api/V1/UpdateRegistrationFieldTemplateRequest.php

@@ -30,7 +30,6 @@ final class UpdateRegistrationFieldTemplateRequest extends FormRequest
             'is_filterable' => ['nullable', 'boolean'],
             'is_portal_visible' => ['nullable', 'boolean'],
             'is_admin_only' => ['nullable', 'boolean'],
-            'section' => ['nullable', 'string', 'max:100'],
             'help_text' => ['nullable', 'string', 'max:5000'],
             'sort_order' => ['nullable', 'integer', 'min:0'],
             'display_width' => ['sometimes', Rule::in(array_column(FieldDisplayWidth::cases(), 'value'))],