Whether or not your employer can claim ownership in court depends entirely on your contract. But no matter what's in your contact, that route is probably not your best option. From what I understand, you created, in your own time, a project that is valuable to your employer, and that they want to use and develop. You're the expert on this project. Not just that, you designed every aspect of it, popularised it, and yo…
California, for example, does not allow contracts for ownerships of things done on your own time, your own equipment and not related to your employer.