feat: optional fixed viewport background on public pages

Adds background_fixed column, admin checkbox, fixed-position layers on the public layout, Dutch strings, and store tests.

Made-with: Cursor

database/migrations/2026_04_04_200000_add_background_fixed_to_preregistration_pages.php

@@ -0,0 +1,24 @@
+<?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('preregistration_pages', function (Blueprint $table): void {
+            $table->boolean('background_fixed')->default(false)->after('background_overlay_opacity');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('preregistration_pages', function (Blueprint $table): void {
+            $table->dropColumn('background_fixed');
+        });
+    }
+};