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/Enums/FieldDisplayWidth.php

@@ -32,7 +32,8 @@ enum FieldDisplayWidth: string
             RegistrationFieldType::RADIO,
             RegistrationFieldType::CHECKBOX,
             RegistrationFieldType::MULTISELECT,
-            RegistrationFieldType::TAG_PICKER => self::FULL,
+            RegistrationFieldType::TAG_PICKER,
+            RegistrationFieldType::HEADING => self::FULL,
         };
     }
 }