Checkout now branches on payment-methods.provider: Überweisung stays
immediate/unchanged, Kreditkarte/PayPal creates a pending_payment order,
mounts Stripe's Payment Element, and defers invoice/email to a webhook-
verified confirm-payment call once the backend actually confirms payment.
Includes a PAYMENT_TEST_MODE mock provider so the whole gated pipeline is
exercisable locally without a real Stripe account.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Marco
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