Create a PaymentIntent on the customer's SAVED DEFAULT card — the one-click top-up path. The server resolves the
default payment method (client can't inject one), pins the PI to it, and returns the client secret; the browser
confirms (3DS in-page if needed). Returns null when there's no default card to charge. No setup_future_usage — the
card is already saved.
Create a PaymentIntent on the customer's SAVED DEFAULT card — the one-click top-up path. The server resolves the default payment method (client can't inject one), pins the PI to it, and returns the client secret; the browser confirms (3DS in-page if needed). Returns null when there's no default card to charge. No setup_future_usage — the card is already saved.