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/bestellungen/[orderNumber]/components/SwitchPaymentButton.tsx

@@ -0,0 +1,73 @@
+"use client";
+
+import { useState } from "react";
+import { PaymentStep } from "../../../../checkout/components/PaymentStep";
+
+// Shown only for a still-unpaid Überweisung order (see page.tsx's own
+// eligibility check, mirroring api/account/orders/[orderNumber]/
+// switch-to-stripe/route.ts's authoritative one) — lets a customer switch
+// to Kreditkarte/PayPal instead of waiting on their own bank transfer.
+// Reuses PaymentStep (the exact same Stripe collection UI checkout uses)
+// once this endpoint hands back a clientSecret — the order already
+// exists, this only changes how it gets paid.
+export function SwitchPaymentButton({ orderNumber }: { orderNumber: string }) {
+  const [state, setState] = useState<
+    | { step: "idle" }
+    | { step: "loading" }
+    | { step: "error"; reason: string }
+    | { step: "paying"; clientSecret: string; orderId: number; testMode: boolean; providerReference?: string }
+  >({ step: "idle" });
+
+  async function start() {
+    setState({ step: "loading" });
+    try {
+      const res = await fetch(`/api/account/orders/${encodeURIComponent(orderNumber)}/switch-to-stripe`, {
+        method: "POST",
+      });
+      const data = await res.json();
+      if (!data.ok) {
+        setState({ step: "error", reason: data.reason || "Umstellung fehlgeschlagen." });
+        return;
+      }
+      setState({
+        step: "paying",
+        clientSecret: data.clientSecret,
+        orderId: data.orderId,
+        testMode: Boolean(data.testMode),
+        providerReference: data.providerReference,
+      });
+    } catch {
+      setState({ step: "error", reason: "Umstellung gerade nicht möglich." });
+    }
+  }
+
+  if (state.step === "paying") {
+    return (
+      <div className="flex flex-col gap-4 w-full border border-border rounded-md p-5">
+        <p className="font-semibold text-body-sm text-text-primary">Mit Kreditkarte/PayPal bezahlen</p>
+        <PaymentStep
+          clientSecret={state.clientSecret}
+          orderNumber={orderNumber}
+          orderId={state.orderId}
+          testMode={state.testMode}
+          providerReference={state.providerReference}
+          returnContext="account"
+        />
+      </div>
+    );
+  }
+
+  return (
+    <div className="flex flex-col gap-2 items-start">
+      <button
+        type="button"
+        onClick={start}
+        disabled={state.step === "loading"}
+        className={`px-5 py-3 rounded-sm border border-border hover:border-brand font-bold text-body-sm text-text-primary transition-colors ${state.step === "loading" ? "opacity-70 pointer-events-none" : ""}`}
+      >
+        {state.step === "loading" ? "…" : "Zahlungsart ändern"}
+      </button>
+      {state.step === "error" && <p className="text-label text-red-600">{state.reason}</p>}
+    </div>
+  );
+}

app/konto/bestellungen/[orderNumber]/page.tsx

@@ -7,11 +7,13 @@ import { Footer } from "../../../components/Footer";
 import { VatBreakdown } from "../../../components/VatBreakdown";
 import { formatPrice, formatDate } from "../../../lib/format";
 import { getSessionCustomer, getCustomerOrderDetail, customerOrderAction } from "../../../lib/customerAuth";
-import { getProductImagesByIds } from "../../../lib/payload";
+import { getProductImagesByIds, getPaymentMethods, groupPaymentMethodsForCheckout } from "../../../lib/payload";
 import { computeTaxBreakdown } from "@einfach-produktiv/invoicing";
 import { buildTrackingUrl, CARRIER_LABELS } from "../../../lib/tracking";
 import { OrderActionButton } from "./components/OrderActionButton";
+import { SwitchPaymentButton } from "./components/SwitchPaymentButton";
 import { OrderStatusBadge } from "../../components/OrderStatusBadge";
+import { PaymentStatusBadge } from "../../components/PaymentStatusBadge";
 
 // Dynamic (was a static "Bestelldetails" title despite this being a
 // per-order route) — just formats the already-known order number into
@@ -47,6 +49,13 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
   const action = customerOrderAction(order.status);
   const imagesByProductId = await getProductImagesByIds(order.items.map((item) => item.product));
   const taxBreakdown = computeTaxBreakdown(order.items, order.subtotal, order.discountAmount, order.shippingCost);
+  // Same "Online-Zahlung" grouping/eligibility the switch-to-stripe route
+  // itself re-checks authoritatively — only offer the button when it
+  // would actually succeed.
+  const canSwitchPayment =
+    order.paymentProvider === "manual" &&
+    order.status === "received" &&
+    groupPaymentMethodsForCheckout(await getPaymentMethods()).some((m) => m.provider === "stripe");
 
   return (
     <>
@@ -73,8 +82,14 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
               <p className="text-label text-text-muted">Zahlungsart</p>
               <p className="text-body-sm text-text-primary">{order.paymentMethodTitle}</p>
             </div>
+            <div className="flex flex-col gap-1">
+              <p className="text-label text-text-muted">Zahlungsstatus</p>
+              <PaymentStatusBadge paymentStatus={order.paymentStatus} />
+            </div>
           </div>
 
+          {canSwitchPayment && <SwitchPaymentButton orderNumber={order.orderNumber} />}
+
           {order.trackingNumber && (
             <div className="flex flex-col gap-1 w-full">
               <p className="text-label text-text-muted">Sendungsverfolgung{order.carrier ? ` (${CARRIER_LABELS[order.carrier] ?? order.carrier})` : ""}</p>

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>
+  );
+}