feat(invoice): informal German tone + show payment method and status

- i18n.de: switch Sie/Ihren to du/dein for salutation, thank-you line,
  customer-VAT label and payment-terms paragraph. Closing line was
  already informal.
- i18n: add paymentMethodLabel/paymentStatusLabel + per-status labels
  (paid/unpaid/partial/refunded) for both DE and EN, plus
  derivePaymentStatus helper that condenses Shopify's
  displayFinancialStatus (PAID, PARTIALLY_PAID, REFUNDED, …) into a
  4-value enum.
- loadOrderForInvoice: query Order.paymentGatewayNames and propagate it
  on the raw view-model.
- composeInvoice + types: expose paymentStatus + paymentGatewayNames on
  InvoiceViewModel (filtered/trimmed). loadDraftOrderForOffer keeps
  paymentGatewayNames empty (drafts have no gateway yet).
- InvoiceDocument: render two new meta rows on real invoices —
  'Zahlart / Payment method' (joined, prettified gateway names) and
  'Zahlstatus / Payment status' (translated label). Storno + offer kinds
  intentionally omit them.
- scripts/render-sample.ts: extend smoke checks to assert the informal
  DE wording, the new payment-method/status rows and the
  paymentStatus/paymentGatewayNames composer outputs.

scripts/render-sample.ts

@@ -134,6 +134,7 @@ function buildAtB2BOrder(): RawOrderForInvoice {
     processedAt: "2026-04-15T10:00:00Z",
     currencyCode: "EUR",
     displayFinancialStatus: "PENDING",
+    paymentGatewayNames: ["manual"],
     taxesIncluded: false,
     customer: {
       firstName: "Lukas",
@@ -278,6 +279,8 @@ async function main() {
   assertEq("no notices for AT B2B with VAT charged", vm.notices.length, 0);
   assert("due date 14 days after invoice date", !!vm.dueDate
     && Math.round((vm.dueDate.getTime() - vm.invoiceDate.getTime()) / 86400000) === 14);
+  assertEq("paymentGatewayNames propagated", vm.paymentGatewayNames.join(","), "manual");
+  assertEq("paymentStatus derived from displayFinancialStatus=PENDING", vm.paymentStatus, "unpaid");
 
   console.log("• EU B2B reverse-charge notice");
   const euOrder = buildEuB2BReverseChargeOrder();
@@ -398,6 +401,21 @@ async function main() {
     !/Thank you for your purchase\.[\s\S]{0,40}Gerhard Berger/.test(enText),
   );
 
+  // Informal German tone (du/dein) — make sure no formal "Sie/Ihren" remains
+  // in the strings we control (footer / signature lines come from settings).
+  assert("DE PDF uses informal salutation 'Hallo,'", deText.includes("Hallo,"));
+  assert("DE PDF no longer uses 'Sehr geehrte Damen und Herren'", !deText.includes("Sehr geehrte Damen und Herren"));
+  assert("DE PDF uses informal 'deine Bestellung'", deText.includes("deine Bestellung"));
+  assert(
+    "DE PDF payment-terms uses informal 'überweise … für dich'",
+    deText.includes("Bitte überweise") && deText.includes("für dich"),
+  );
+  assert("DE PDF shows payment status row", deText.includes("Zahlstatus"));
+  assert("DE PDF shows payment status value 'Offen' for PENDING", deText.includes("Offen"));
+  assert("DE PDF shows payment method row", deText.includes("Zahlart"));
+  assert("EN PDF shows payment status row", enText.includes("Payment status"));
+  assert("EN PDF shows payment status value 'Outstanding' for PENDING", enText.includes("Outstanding"));
+
   // Fallback: when footerNoteEn is empty, English uses the German note.
   console.log("• Footer note fallback (en → de when EN empty)");
   const settingsNoEn = { ...(settings as object), footerNoteEn: "" } as never;