An idea is just that  an idea. Legally, ideas aren't typically protected under copyright laws. However, app builders typically copyright their app idea as soon as you start to code the project and there's something tangible that can be protected.

Ideas alone are not protected under intellectual property law. There are two primary ways that you would be able to sue the company for stealing your idea. The first is if you did, in fact, reduce the idea to a protectable form before telling the company about it.