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/lib/emailTemplates.ts

@@ -74,7 +74,7 @@ function escapeHtml(s: string): string {
 // this template (16px) — not squeezed into the narrow Gesamtsumme table
 // like an initial draft of the invoice version was before that got
 // widened per feedback.
-function vorkasseNotice(orderNumber: string, seller: CompanySettings | null): string {
+function vorkasseNotice(orderNumber: string, seller: CompanySettings | null, hasOnlinePaymentOption: boolean): string {
   const bankLine = seller && (seller.iban || seller.bic)
     ? [seller.bankName, seller.iban && `IBAN ${seller.iban}`, seller.bic && `BIC ${seller.bic}`].filter(Boolean).join(" · ")
     : null;
@@ -85,6 +85,7 @@ function vorkasseNotice(orderNumber: string, seller: CompanySettings | null): st
           <p style="margin:0 0 8px;">Bitte überweise den Rechnungsbetrag unter Angabe der Bestellnummer ${escapeHtml(orderNumber)} auf ${bankLine ? "folgende Bankverbindung:" : "die dir genannte Bankverbindung."}</p>
           ${bankLine ? `<p style="margin:0 0 12px;font-weight:700;color:${TEXT_PRIMARY};">${escapeHtml(bankLine)}</p>` : ""}
           <p style="margin:0;">Deine Bestellung wird nach Zahlungseingang bearbeitet (in der Regel innerhalb von 1–2 Werktagen).</p>
+          ${hasOnlinePaymentOption ? `<p style="margin:8px 0 0;">Zahlungsart geändert? Solange die Überweisung noch nicht bei uns eingegangen ist, kannst du in deinem Konto jederzeit auf Kreditkarte/PayPal umsteigen.</p>` : ""}
         </td>
       </tr>
     </table>
@@ -215,6 +216,12 @@ export type OrderConfirmationData = {
   // fragile thing a payment-methods rename already broke once this
   // session (see @einfach-produktiv/invoicing's isPaidImmediately()).
   isManualPayment: boolean;
+  // Only meaningful when isManualPayment is true — whether at least one
+  // Stripe-backed payment method is currently active, so the Vorkasse
+  // notice can mention the option to switch instead of promising it
+  // unconditionally. Set by the checkout route from the same
+  // getPaymentMethods() call it already makes.
+  hasOnlinePaymentOption?: boolean;
 };
 
 export const SAMPLE_ORDER: OrderConfirmationData = {
@@ -298,7 +305,7 @@ export function renderOrderConfirmationHtml(template: EmailTemplateContent, orde
       </tr>
       ${taxRows}
     </table>
-    ${order.isManualPayment ? vorkasseNotice(order.orderNumber, seller) : ""}
+    ${order.isManualPayment ? vorkasseNotice(order.orderNumber, seller, Boolean(order.hasOnlinePaymentOption)) : ""}
   `;
 
   return emailShell("✓", escapeHtml(template.heading), body, template.footerText, buildLegalFooterLines(seller));