add set layout api

backend/app/Set/UseCases/GetSetLayout/ElementLayoutNode.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Set\UseCases\GetSetLayout;
+
+use App\Element\Element;
+
+final readonly class ElementLayoutNode
+{
+    /**
+     * @param list<ElementLayoutNode> $children
+     */
+    public function __construct(
+        private Element $element,
+        private array $children,
+    ) {}
+
+    public function getElement(): Element
+    {
+        return $this->element;
+    }
+
+    /**
+     * @return list<ElementLayoutNode>
+     */
+    public function getChildren(): array
+    {
+        return $this->children;
+    }
+}