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/tests/Feature/RegistrationFieldTemplate/RegistrationFieldTemplateTest.php

@@ -152,7 +152,7 @@ class RegistrationFieldTemplateTest extends TestCase
             ->where('is_system', true)
             ->count();
 
-        $this->assertEquals(11, $count);
+        $this->assertEquals(16, $count);
     }
 
     public function test_create_field_from_template(): void