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/Models/RegistrationFieldTemplate.php

@@ -34,7 +34,6 @@ final class RegistrationFieldTemplate extends Model
         'is_filterable',
         'is_portal_visible',
         'is_admin_only',
-        'section',
         'help_text',
         'sort_order',
         'display_width',

api/app/Models/RegistrationFormField.php

@@ -38,7 +38,6 @@ final class RegistrationFormField extends Model
         'is_portal_visible',
         'is_admin_only',
         'is_filterable',
-        'section',
         'help_text',
         'sort_order',
         'display_width',