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

@@ -21,10 +21,13 @@ 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. 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") {
+  // checked here too for a friendly error instead of a bare 409. An
+  // explicit allowlist ('pending'/'not_applicable'), not a
+  // paymentStatus !== "paid" blocklist — see switchPaymentToStripe.ts's
+  // own comment on why 'failed'/'refunded'/'partially_refunded' aren't
+  // meaningful states to switch from either.
+  const eligiblePaymentStatus = order.paymentStatus === "pending" || order.paymentStatus === "not_applicable";
+  if (order.paymentProvider !== "manual" || order.status !== "received" || !eligiblePaymentStatus) {
     return NextResponse.json({ ok: false, reason: "Die Zahlungsart kann für diese Bestellung gerade nicht geändert werden." }, { status: 400 });
   }