enforce element level hierarchy

backend/app/Element/CreateElementDto.php

@@ -2,14 +2,13 @@
 
 namespace App\Element;
 
-use App\Set\Set;
+use App\Set\SetLevel;
 
 final readonly class CreateElementDto
 {
     public function __construct(
-        public Set $set,
         public string $name,
-        public string $kind,
+        public SetLevel $level,
         public ?Element $parentElement,
     ) {}
 }