fix: vuex mutation violation from draggable (#674)

Co-authored-by: Dominik Pschenitschni <mail@celement.de>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/674
Reviewed-by: konrad <k@knt.li>
Co-authored-by: dpschen <dpschen@noreply.kolaente.de>
Co-committed-by: dpschen <dpschen@noreply.kolaente.de>

src/services/list.js

@@ -39,6 +39,11 @@ export default class ListService extends AbstractService {
 		return list
 	}
 
+	update(model) {
+		const newModel = { ... model }
+		return super.update(newModel)
+	}
+
 	background(list) {
 		if (list.background === null) {
 			return Promise.resolve('')