FR (Authenticator): Dismiss button for modal variant

### On which framework/platform would you like to see this feature implemented?

React

### Which UI component is this feature-request for?

Authenticator

### Please describe your feature-request in detail.

I was looking to have a dismiss button on the Authenticator and couldn't figure out how to add it

That's my best shot :)
It works except the double border between component and the additional box


### Please describe a solution you'd like.

Is it possible to make it prettier with current design and override capabilities?

Would it make sense to add the cancel button as part of the component itself?

Thank you!

### We love contributors! Is this something you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request.
- [ ] ⚠️ This feature might incur a breaking change.

Start by reviewing the React Authenticator's modal variant and its current design and override capabilities. The issue does not name files or tests; completion would require an agreed dismiss-button behavior and a polished result without the reported double border.

Written by the indexing model from the issue text.