How to implement storefront login with user data fetched from SSE?

Summary:

We currently have Storefront SSO with SAML implemented that customers use to login. We want to implement a custom login without SSO.

Content (please ensure you mask any confidential information):

We don't want the user to leave the Storefront for the login process. We want to display a form in a popup/modal that they can enter their username/password in. Clicking login, we want to call a custom SSE endpoint that validates and authenticates the user and returns back with all of the customer details.

Version (include the version you are using, if applicable):

Storefront Classic 25.0.3

Code Snippet (add any code snippets that support your topic, if applicable):