Different contributor here. Please permit me to assist.
Based upon what you have described, the issue boils down to whether or not you work as a software developer for your employer during your regular employment.
If you do, then pursuant to the U.S. Copyright Act section 101, your work is a "work made for hire," and the employer owns the copyright, regardless of whether or not you are the sole creator of the application. In return, your employer owes you wages for your hours of employment, or salary, if you are salary exempt under California law as a software developer (paid at least $39.90 per hour).
If you are not employed as a software developer for your employer, and you used all of your own equipment and developed the application during your non-working hours, then you are an independent contractor and since there was no written agreement, then under the U.S. Copyright Act you own the copyright, which means that while you may have given the employer an implied license for one copy of the software, you have not necessarily licensed the work for distribution to third parties.
This further means that you are entitled to sue the employer for copyright infringement
That's the good news. The bad news is that a copyright lawsuit must be brought in U.S. District Court
. There is no "small claims
action" available. You will need to hire a copyright litigation
attorney and let him/her handle the case -- because this is the farthest thing from a "do-it-yourself" legal action that exists.
For a competent copyright litigation attorney referral, see this link.
Note: you must also register the software application with the U.S. Copyright Office.
Otherwise, the court will dismiss the case, or stay it until you do file -- and you will not be entitled to statutory damages (which can be $750 per unauthorized copy, regardless of actual damages).
Please let me know if I can be of further assistance.