Two live payment demos, wires cut

Shipped today: both Mexican card gateways I work with now render on this site, live, with the wires cut. Clip's embedded checkout and Mercado Pago's CardPayment Brick, side by side, in demo mode. Nothing moves a peso.

The bug worth logging: Clip's checkout.js registers its listener on DOMContentLoaded. Inject the script after React mounts and that event has already fired, so the script never finds its button. Dead button, no error, no clue. The fix is rendering the script and the button in the initial server-side HTML. Also disabled installments on the Brick; a demo does not need month-by-month plans.

The full comparison of the two philosophies (hand off the charge vs. own it) is in the Notes board: Two ways to take a card payment in Mexico.