Checkout & payment flows

New checkout had to behave like the old one. It did not. Type mismatches in payment response mapping caused failures after a successful charge. Guest checkout broke when user_id was JSON null instead of missing.

What I did

• Traced post-payment 500s back to mapping, not the gateway itself.
• Fixed guest checkout null handling.
• Aligned iframe, wallet UI, and wallet-pay paths with legacy behaviour and retested card, guest, and Apple Pay.