Restrict Rücksendung anfragen to delivered orders, fix Impressum email link

- customerOrderAction() only offers "Rücksendung anfragen" once an order
  is delivered, not already at shipped — a return before the package
  arrived doesn't make sense yet. UI-only change (a stricter subset of
  what the backend's CUSTOMER_ALLOWED_TRANSITIONS already permits).
- AnbieterAngaben.tsx's seller email is now a real mailto: link — it was
  plain text, the only non-clickable email on the site.

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

@@ -55,7 +55,7 @@ export default async function KontoBestellungDetailPage({ params }: { params: Pr
   const canSwitchPayment =
     order.paymentProvider === "manual" &&
     order.status === "received" &&
-    order.paymentStatus !== "paid" &&
+    (order.paymentStatus === "pending" || order.paymentStatus === "not_applicable") &&
     groupPaymentMethodsForCheckout(await getPaymentMethods()).some((m) => m.provider === "stripe");
 
   return (