Add a Stripe Payment Method

Adding a payment method touches at least five places across PHP and frontend. The most common pitfall in this codebase is missing one of them — the method either doesn't appear on checkout at all, or appears on one surface but not another.

Required changes (in order)

1. PHP class

Create includes/payment-methods/class-wc-stripe-upe-payment-method-<id>.php extending WC_Stripe_UPE_Payment_Method.

Required overrides at minimum:

• get_id() — return the Stripe payment method type (e.g., klarna, affirm, sepa_debit).
• get_label() — translated method name shown to merchants in admin.
• get_icon() — return the icon URL registered in step 4.
• Capability flags — is_subscription_supported(), is_pre_order_supported(), currency/country constraints.

If the method supports recurring payments, also include WC_Stripe_Subscriptions_Trait and ensure subscription/pre-order initialization runs in the gateway constructor. Reference an existing recurring-capable method (Klarna, Card, SEPA) for the trait wiring pattern.