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/database/migrations/2026_04_16_142626_remove_section_from_registration_fields_tables.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('registration_field_templates', function (Blueprint $table) {
+            $table->dropColumn('section');
+        });
+
+        Schema::table('registration_form_fields', function (Blueprint $table) {
+            $table->dropColumn('section');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('registration_field_templates', function (Blueprint $table) {
+            $table->string('section', 100)->nullable()->after('is_admin_only');
+        });
+
+        Schema::table('registration_form_fields', function (Blueprint $table) {
+            $table->string('section', 100)->nullable()->after('is_filterable');
+        });
+    }
+};