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/challenge/components/EmailCapture.tsx

@@ -13,11 +13,11 @@ function LockIcon() {
 }
 
 export function EmailCapture({ buttonLabel = "Challenge starten" }: { buttonLabel?: string }) {
-  const { email, emailError, consent, setConsent, status, error, emailRef, handleEmailChange, handleEmailBlur, handleSubmit } =
+  const { email, emailError, consent, setConsent, status, error, successMessage, emailRef, handleEmailChange, handleEmailBlur, handleSubmit } =
     useNewsletterSignup("challenge");
 
   if (status === "success") {
-    return <p className="text-[1rem] text-[#222221] font-medium">Fast geschafft! Schau kurz in dein Postfach – da wartet schon eine Mail von uns.</p>;
+    return <p className="text-[1rem] text-[#222221] font-medium">{successMessage}</p>;
   }
 
   return (