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/Resources/Api/V1/RegistrationFieldTemplateResource.php

@@ -24,7 +24,6 @@ final class RegistrationFieldTemplateResource extends JsonResource
             'is_filterable' => $this->is_filterable,
             'is_portal_visible' => $this->is_portal_visible,
             'is_admin_only' => $this->is_admin_only,
-            'section' => $this->section,
             'help_text' => $this->help_text,
             'sort_order' => $this->sort_order,
             'display_width' => $this->display_width->value,

api/app/Http/Resources/Api/V1/RegistrationFormFieldResource.php

@@ -26,7 +26,6 @@ final class RegistrationFormFieldResource extends JsonResource
             'is_portal_visible' => $this->is_portal_visible,
             'is_admin_only' => $this->is_admin_only,
             'is_filterable' => $this->is_filterable,
-            'section' => $this->section,
             'help_text' => $this->help_text,
             'sort_order' => $this->sort_order,
             'display_width' => $this->display_width->value,