Add createCheckout mutation; simulate Stripe redirect

## Goal
Add a `createCheckout` mutation that POSTs the order to a mock endpoint and simulates a Stripe redirect (e.g., a brief delay then navigate to `/checkout/success`).

## Acceptance criteria
- [ ] Submit button calls the mutation
- [ ] Submit disabled while in flight
- [ ] On success, navigate to `/checkout/success`
- [ ] On error, show an error toast and keep the form editable

Start by locating the checkout form submit handler and the createCheckout mutation entry point, then trace how the mock endpoint, redirect, and error toast are handled. It is done when submission is wired to the mutation, the button is disabled in flight, success navigates to /checkout/success, and errors leave the form editable.

Written by the indexing model from the issue text.