just say to her, regardless of whether she was willing to cover the costs, using your car, without permission is not acceptable, and you don't want her to do it again.
If she paid to fix your car, it's still been damaged. It's still lost value, and actually her damaging the car one of the best case scenario is that could've happened. She could've damaged someone else's car. She could've damaged someone else's property. She could've hurt somebody. She even could've killed somebody.
Driving without proper insurance is absolutely out of line whatever the scenario.
In my view, using all of the bath bombs would be a bit shit, but using some/the majority is a bit meh.
You highlight all the things she gets out this arrangement, in that is use of the bath et cetera. You perhaps need to remind yourself that she is looking after your cat, she is there for your cat, so it's not on its own, and she is present in your house looking after it.
If you do this again you need to be absolutely explicit about the car, and pack away things that you don't want her to use.