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/lib/customerAuth.ts

@@ -523,6 +523,10 @@ export async function getCustomerOrders(token: string, customerId: number, exclu
 
 export type CustomerOrderDetail = CustomerOrder & {
   id: number;
+  // 'manual' (Überweisung) vs 'stripe' (Kreditkarte/PayPal) — see
+  // api/account/orders/[orderNumber]/switch-to-stripe/route.ts, which only
+  // offers a payment-method switch for a still-'manual' order.
+  paymentProvider: "manual" | "stripe";
   // 'not_applicable' for Überweisung orders (never gated); see
   // spicy-leaping-pizza.md §1 — read by /api/checkout/status for the
   // post-Stripe-redirect polling page.