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/checkout/components/PaymentStep.tsx

@@ -24,25 +24,43 @@ type Props = {
   testMode: boolean;
   /** Only present in test mode — see api/checkout/route.ts's own comment. */
   providerReference?: string;
+  /** Where /checkout/verarbeitung sends the customer once payment is
+   * confirmed — "checkout" (default) clears the cart/draft and lands on
+   * /bestellbestaetigung, exactly like today. "account" is used by the
+   * account order detail page's "Zahlungsart ändern" flow (an existing,
+   * already-confirmed order — nothing to clear, and /bestellbestaetigung
+   * would be the wrong destination): lands back on that same order's
+   * page instead. See VerarbeitungContent.tsx's own branching on this. */
+  returnContext?: "checkout" | "account";
 };
 
 // Rendered by CheckoutContent once /api/checkout returns
 // `requiresPayment: true` (Kreditkarte/PayPal) — see
 // spicy-leaping-pizza.md §3/§7. The order already exists in Payload at
 // this point (status 'pending_payment'); this step only collects/confirms
-// the actual payment, it doesn't create anything.
-export function PaymentStep({ clientSecret, orderNumber, orderId, testMode, providerReference }: Props) {
+// the actual payment, it doesn't create anything. Also reused as-is by
+// the account order detail page's payment-method-switch flow (see
+// returnContext above) — the Stripe collection UI itself is identical
+// either way, only the post-payment destination differs.
+export function PaymentStep({ clientSecret, orderNumber, orderId, testMode, providerReference, returnContext = "checkout" }: Props) {
   if (testMode) {
-    return <TestPaymentButtons orderNumber={orderNumber} orderId={orderId} providerReference={providerReference ?? ""} />;
+    return (
+      <TestPaymentButtons
+        orderNumber={orderNumber}
+        orderId={orderId}
+        providerReference={providerReference ?? ""}
+        returnContext={returnContext}
+      />
+    );
   }
   return (
     <Elements stripe={getStripe()} options={{ clientSecret }}>
-      <StripePaymentForm orderNumber={orderNumber} />
+      <StripePaymentForm orderNumber={orderNumber} returnContext={returnContext} />
     </Elements>
   );
 }
 
-function StripePaymentForm({ orderNumber }: { orderNumber: string }) {
+function StripePaymentForm({ orderNumber, returnContext }: { orderNumber: string; returnContext: "checkout" | "account" }) {
   const stripe = useStripe();
   const elements = useElements();
   const [submitting, setSubmitting] = useState(false);
@@ -62,7 +80,7 @@ function StripePaymentForm({ orderNumber }: { orderNumber: string }) {
     const { error: confirmError } = await stripe.confirmPayment({
       elements,
       confirmParams: {
-        return_url: `${window.location.origin}/checkout/verarbeitung?orderNumber=${encodeURIComponent(orderNumber)}`,
+        return_url: `${window.location.origin}/checkout/verarbeitung?orderNumber=${encodeURIComponent(orderNumber)}&context=${returnContext}`,
       },
     });
     // Only reached for immediate client-side failures (e.g. invalid card
@@ -88,7 +106,17 @@ function StripePaymentForm({ orderNumber }: { orderNumber: string }) {
   );
 }
 
-function TestPaymentButtons({ orderNumber, orderId, providerReference }: { orderNumber: string; orderId: number; providerReference: string }) {
+function TestPaymentButtons({
+  orderNumber,
+  orderId,
+  providerReference,
+  returnContext,
+}: {
+  orderNumber: string;
+  orderId: number;
+  providerReference: string;
+  returnContext: "checkout" | "account";
+}) {
   const router = useRouter();
   const [submitting, setSubmitting] = useState<"paid" | "failed" | null>(null);
   const [error, setError] = useState<string | null>(null);
@@ -108,7 +136,7 @@ function TestPaymentButtons({ orderNumber, orderId, providerReference }: { order
         setSubmitting(null);
         return;
       }
-      router.push(`/checkout/verarbeitung?orderNumber=${encodeURIComponent(orderNumber)}`);
+      router.push(`/checkout/verarbeitung?orderNumber=${encodeURIComponent(orderNumber)}&context=${returnContext}`);
     } catch {
       setError("Testzahlung konnte nicht ausgeführt werden.");
       setSubmitting(null);