Getting checkout ready for Strong Customer Authentication

When Strong Customer Authentication became mandatory, every card flow on the platform had to change. We reworked the checkout so hashing uses each merchant's own gateway secret rather than a shared one, and rebuilt it to match each provider's specification exactly.

We also moved Stripe payments onto Checkout Sessions so 3-D Secure challenges are handled properly, and kept the other card gateways on their hosted payment pages, where the acquirer runs the authentication step — keeping the checkout compliant as the rules keep tightening.