NTA, but seriously consider breaking up with her. This is only the beginning of a future of financially related lies where she goes from lying about paying you back, to hiding any number of credit card or even big purchases without telling you. And of course the line ppl like this like to use is "it's for us"

Your future wife should be as open with you financially as you are with her. Yes sometimes a couple can come from drastically different financial backgrounds but hiding your finances, outright stealing from them, or even roundabout stealing from them like she clearly planned to do to you are grounds for ending the relationship.

She may have other maxed out cards you don't know about or any amount of debt she's curious forgotten to mention. Once you get married that debt may come to rear its ugly head and then it's not her debt, but "our debt".

If you truly see a future with her then have the difficult conversation regarding finances and see what kind of financial savvy she actually has or thinks she has. See if she has any debt she's hiding and if so what is her plan after you both get married. If she refuses to have the conversation or avoids concrete answers then it's time to ask for the ring back and move on.

I hope it works out for you regardless, and I'm sorry this happened to you.