[FR] Authenticator: hideSignIn

### 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 want a `hideSignIn` flag so that:
- I can use the Authenticator for _only_ sign up on some pages
- I can use the Authenticator for _only_ sign in on some pages (this already exists, `hideSignUp`)
- if I want sign up and sign in, I can have more control over the appearance (eg by toggling `hideSignIn` and `hideSignUp` and using the `useAuthenticator` transition functions)

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

`` (with `initialState` being set automatically to `signUp`)

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

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

Start by locating the React Authenticator implementation and tracing the existing hideSignUp handling, including the initialState and useAuthenticator entry points. Done means hideSignIn supports the requested sign-up-only and sign-in-only configurations while preserving existing behavior, with coverage for the new flag.

Written by the indexing model from the issue text.