After cart: implement checkout login intercept page

[KFie]

Goal

Enhance the checkout process by offering guest customers to either log in or create a new account.

Requirements

If a customer is not logged in, has items in their cart and clicks "Proceed to checkout", they arrive on an intercept page offering three options:

• Log in with existing user account
• Create user account
• Check out as guest user

Layout

Page should have a reduced header, consisting of store logo and a link back to the cart. Options to proceed should display in three tiles, possibly using this layout (on desktop):

User Flows

• Log in with existing account: after providing valid credentials, user gets sent to shipping step (we currently don't skip steps despite the options to store addresses and payment data); providing invalid credentials results in error messaging, user canot proceed.

• Create user account: after providing valid data, account is created, user possibly sees a success message and gets sent to shipping step.

• Check out as guest user: when clicking to proceed, user is sent to shipping step.