Add schema.org structured data, Vorkasse email notice, newsletter duplicate detection

- Organization (site-wide), Product (/todo-cards), BlogPosting (every
  /blog/[slug]) JSON-LD via new app/lib/structuredData.ts — no new
  Payload fields needed, derived from existing data. Verified locally
  by curling each page and checking the rendered script tag.
- Order confirmation email gains the same "please transfer to this
  account, processed after payment received" notice the invoice PDF
  already had for Vorkasse orders — OrderConfirmationData's new
  isManualPayment flag is set explicitly by each caller (never derived
  from paymentMethodTitle, which already broke once this session after
  a payment-methods rename). CompanySettings gains bankName (existed on
  the backend, was missing from the frontend's type/usage).
- Newsletter signup now detects an already-subscribed email
  (verified empirically: Brevo's doubleOptinConfirmation endpoint gives
  identical 201 responses for new vs. already-confirmed contacts) via a
  GET /v3/contacts/{email} pre-check, and shows a distinct message
  instead of silently resending the confirmation mail. Success message
  text centralized in useNewsletterSignup.ts instead of duplicated
  across 4 forms.
- Bumped @einfach-produktiv/invoicing to the version with the
  unpaid-notice layout fix (full width, more top spacing — was
  squeezed into the narrow paid-badge column).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/todo-cards/page.tsx

@@ -7,7 +7,8 @@ import { Pricing } from "./components/Pricing";
 import { Footer } from "../components/Footer";
 import { TestimonialsGrid } from "../components/TestimonialsGrid";
 import { LiveTestimonialsGrid } from "../components/LiveTestimonialsGrid";
-import { getTestimonials } from "../lib/payload";
+import { getTestimonials, getProductBySlug, getCompanySettings } from "../lib/payload";
+import { buildProductSchema } from "../lib/structuredData";
 
 const title = "ToDo-Karten – Kleine Karten. Große Wirkung.";
 const description =
@@ -34,10 +35,18 @@ export const metadata: Metadata = {
 
 export default async function TodoCardsPage() {
   const { isEnabled: isPreview } = await draftMode();
-  const testimonials = await getTestimonials("todo-cards", { draft: isPreview });
+  const [testimonials, product, seller] = await Promise.all([
+    getTestimonials("todo-cards", { draft: isPreview }),
+    getProductBySlug("todo-karten"),
+    getCompanySettings(),
+  ]);
+  const productSchema = product ? buildProductSchema(product, "https://einfach-produktiv.mk360.de/todo-cards", seller) : null;
 
   return (
     <>
+      {productSchema && (
+        <script type="application/ld+json" dangerouslySetInnerHTML={{ __html: JSON.stringify(productSchema) }} />
+      )}
       <main className="flex flex-col flex-1">
         <TodoKartenHero />
         <HowItWorks />