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/konto/components/PaymentStatusBadge.tsx

@@ -0,0 +1,33 @@
+// Same visual pattern as OrderStatusBadge — but answers a different
+// question ("hat Stripe/die Buchhaltung eine Zahlung bestätigt?", not
+// "wo im Fulfillment steht die Bestellung"). 'not_applicable' (an
+// Überweisung order before payment is manually reconciled) reads as
+// "offen", same as a still-'pending' Stripe order — the customer doesn't
+// need to know the internal distinction between the two.
+const LABEL: Record<string, string> = {
+  not_applicable: "Offen",
+  pending: "Offen",
+  paid: "Bezahlt",
+  failed: "Fehlgeschlagen",
+  refunded: "Erstattet",
+  partially_refunded: "Teilweise erstattet",
+};
+
+const STYLES: Record<string, string> = {
+  not_applicable: "bg-bg-muted text-text-muted",
+  pending: "bg-bg-muted text-text-muted",
+  paid: "bg-success-subtle text-success",
+  failed: "bg-red-50 text-red-600",
+  refunded: "bg-bg-muted text-text-light",
+  partially_refunded: "bg-orange-50 text-orange-600",
+};
+
+export function PaymentStatusBadge({ paymentStatus }: { paymentStatus: string }) {
+  return (
+    <span
+      className={`inline-flex items-center px-2.5 py-1 rounded-full text-label font-bold whitespace-nowrap ${STYLES[paymentStatus] ?? "bg-bg-muted text-text-muted"}`}
+    >
+      {LABEL[paymentStatus] ?? paymentStatus}
+    </span>
+  );
+}