What does it say about someone that accepts adding their name to a publication without doing any of the work?

No, it is certainly not unethical. You made an agreement at the beginning and you fulfilled it. She thanked you for your courtesy. All is well. The one, perhaps, contribution that she did make was to get you attached to the project idea, from which you will benefit. You are the primary author, in this case, of course.

There are many reasons for including someone on a paper and not all are benign, but I think this one is. I assume the other person is quite grateful, and maybe even a bit embarrassed. She may even not want to insult you by asking to be removed, since you had the earlier agreement.

But judging her ethics will get you nothing but trouble.

Someone must have had the original idea which became her background notes, it may have even been her idea, so it would be appropriate to still list her as a co-author.

It's not clear where the research idea came from, but also since you took over the research from her and told her that you would add her name to the publication it "costs" you little but gains you more to include her in the author list.

She may not take any action if you were to not include her, and the thankyou could be an expression of appreciation that you included her even though you carried the research through from idea to publication. Again, the long-term goodwill is worth it.

It's a bit cyclical, but because she's an author on the paper, you should (must?) have her read the paper before you submit it.

I assume since she was once involved on the project, she would have an interest in editing the paper beyond rubber-stamping it. Therefore, you can have a meaningful editing round with her, which would increase how much of a contribution she had to the paper, which might make you feel better.

To address your question specifically, you said you didn't use her notes. While I'm somewhat skeptical you didn't look at them and therefore learned nothing at all from them, she doesn't know that (unless you sent her an odd email about how much you didn't use her notes), and so probably assumes she made an intellectual contribution and still deserves authorship. So no, I don't think she is behaving unethically.