Hotfix: Restore skybox property (#28)

Restores the `skyboxId` property that was acccidentally removed from the
`<sts-viewer />` element

studio-frontend/src/app/app.element.ts

@@ -190,6 +190,7 @@ export class AppElement extends LitElement {
 			}
 			<sts-viewer
 				objectId="Mannequin.gltf"
+        skyboxId="test_scene"
 				.mode=${this.mode}
 				@scene-state=${this.onSceneStateChange}
 			></sts-viewer>