FR (Authenticator): Modal variation for Authenticator should be dismissable

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

React, Angular, Vue

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

Authenticator

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

As a developer I would like to be able to cancel the sign-in/sign-up process and return to the app without signing in. Currently the modal variation of the Authenticator is not dismissable.

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

This can be accomplished today by customizing the footer of the SignIn component, example [here](https://stackblitz.com/edit/react-ts-g7aut7?file=App.tsx).

The Authenticator component should support a dismissable-esque prop out of the box for the modal variation.

### 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.

The issue names no files or tests; start by locating the Authenticator modal variation and the SignIn footer customization path referenced in the issue. Review how the modal is implemented across the requested framework integrations, then verify that a dismissable option lets users cancel sign-in or sign-up and return to the app.

Written by the indexing model from the issue text.