Reflect Orders.paymentStatus's new meaning for Überweisung orders

- switch-to-stripe eligibility gained paymentStatus !== "paid" — an
  order an admin already marked paid by hand must never also be
  switchable to Stripe.
- PaymentStatusBadge: "Offen" now renders in the same red/subtle
  style as a failed status (was neutral grey) — worth visually flagging,
  now that it's a real tracked state rather than a permanent placeholder.

app/api/account/orders/[orderNumber]/switch-to-stripe/route.ts

@@ -21,8 +21,10 @@ export async function POST(request: Request, { params }: { params: Promise<{ ord
   if (!order) return NextResponse.json({ ok: false, reason: "Bestellung nicht gefunden." }, { status: 404 });
 
   // Same eligibility the backend endpoint re-checks authoritatively —
-  // checked here too for a friendly error instead of a bare 409.
-  if (order.paymentProvider !== "manual" || order.status !== "received") {
+  // checked here too for a friendly error instead of a bare 409. An admin
+  // can mark an Überweisung order paid by hand (Orders.ts's paymentStatus
+  // field) — that order must never also be switched to Stripe.
+  if (order.paymentProvider !== "manual" || order.status !== "received" || order.paymentStatus === "paid") {
     return NextResponse.json({ ok: false, reason: "Die Zahlungsart kann für diese Bestellung gerade nicht geändert werden." }, { status: 400 });
   }