Don't copy the code directly from any company assets. There are plenty of ways to track code and data theft these days, so don't even attempt it. I am just saying that as a friendly reminder.

Honestly, there is not much that a company can do unless they specially own the business logic of what you are doing. Are there aspects to the code that apply to internal proprietary software? That probably isn't wise to share.

While I am not a lawyer, a general rule of thumb is that if you think you might be stealing something, you probably are. Anything you do on company time, is technically owned by that company.

If your previous work gets discarded by that company, never talk about it again. Never code it again for that company and just let the idea die, as far as that company is concerned. Independently resurrect the idea at a later date.

Nobody here can really tell you what to do, btw. Quite honestly, if you think that you can claim ownership of what you have, pay a few hundred bucks for a consultation with a lawyer.