Keep checkout dialog open for inline card retry on payment failure

Summary

When a card payment fails in the checkout dialog, keep the dialog open and let the customer fix and retry inline — instead of closing and redirecting to the return URL.

Current behaviour

On a failed payment, the checkout dialog closes and the customer is redirected to the return URL, with no opportunity to correct the issue on the spot.

Requested behaviour

On a failed payment, keep the checkout open, show the failure inline, and allow the customer to update/correct their card and retry — until the payment succeeds or they explicitly close the dialog.

Why it matters

Recovers conversions lost at the most critical point of the purchase and matches the retry experience customers expect from other payment providers.