Allow switching an unpaid Überweisung order to Stripe payment

New "Zahlungsart ändern" button on the account order detail page,
shown for a still-'received', still-manual (Überweisung) order when an
active Stripe payment method exists. Reuses PaymentStep (the same Stripe
Payment Element checkout uses) and /checkout/verarbeitung's polling logic
(both now take a returnContext prop/param to land back on the order page
instead of clearing the cart and redirecting to /bestellbestaetigung).

Also:
- Payment status badge (Offen/Bezahlt/...) next to the existing Zahlungsart
  display on the order detail page.
- A one-line mention of the switch option in the Vorkasse unpaid notice
  in the order-confirmation email, shown only when a Stripe option is
  actually active (hasOnlinePaymentOption).
- CustomerOrderDetail gained paymentProvider (was missing from the type
  entirely, even though the field already existed on the order).

Backend counterpart: docker/payload's switchPaymentToStripeEndpoint.

app/api/checkout/route.ts

@@ -1,6 +1,6 @@
 import { NextResponse } from "next/server";
 import type { CartItem } from "../../lib/cart";
-import { getShippingMethods, getPaymentMethods, getCompanySettings } from "../../lib/payload";
+import { getShippingMethods, getPaymentMethods, getCompanySettings, groupPaymentMethodsForCheckout } from "../../lib/payload";
 import { validateDiscountCode, redeemDiscountCode } from "../../lib/discountServer";
 import { createOrder } from "../../lib/orderServer";
 import { getSessionCustomer, registerCustomer, setSessionCookie, type CustomerSummary } from "../../lib/customerAuth";
@@ -467,6 +467,9 @@ export async function POST(request: Request) {
         discountCode: body.discountCode || null,
         total,
         isManualPayment: true,
+        // Only meaningful for the Vorkasse notice above — whether a
+        // switch to Kreditkarte/PayPal is even worth mentioning right now.
+        hasOnlinePaymentOption: groupPaymentMethodsForCheckout(paymentMethods).some((m) => m.provider === "stripe"),
       },
       body.email,
     ).catch((err) => {