The GraphQL Storefront API’s Login Mutation now supports fetching all of the details of the customer account after a successful login. Consider this query:

mutation Login($email: String!, $pass: String!) {
  login(email: $email, password: $pass) {
    result
    customer {
      entityId
      firstName
      lastName
      email
      storeCredit {
        value
        currencyCode
      }
      attributes {
        attribute(entityId: 123) {
          name
          value
        }
      }
    }
  }
}