fix(invoice): partial refund stays "Bezahlt" + use "Endbetrag" final-row label

Two related bugs surfaced when a paid order was partially refunded
(Shopify flips `displayFinancialStatus` to PARTIALLY_REFUNDED as soon
as *any* refund is posted, even a small one):

1. The status row showed "Erstattet" / "Refunded" even though the
   customer paid in full and the merchant kept the difference. The
   correct status is "Bezahlt" / "Paid" — only when the refund equals
   (or, defensively, exceeds) the gross is the order genuinely
   refunded.

2. The final row beneath the new refund block was labelled "Offener
   Betrag" / "Outstanding amount", falsely suggesting the customer
   still owes the kept portion. For an order that has been refunded
   but is no longer owing anything, that row is just the final amount
   the merchant kept — "Endbetrag" / "Total".

Truth table now implemented:

  displayFinancialStatus | refunded     | paymentStatus | final-row label
  -----------------------+--------------+---------------+-----------------
  PAID                   | 0            | paid          | (no refund rows)
  PAID                   | >0           | paid          | Endbetrag
  PARTIALLY_REFUNDED     | < gross      | paid (NEW)    | Endbetrag (NEW)
  PARTIALLY_REFUNDED     | == gross     | refunded      | Endbetrag
  REFUNDED               | == gross     | refunded      | Endbetrag
  PARTIALLY_PAID         | 0            | partial       | (no refund rows)
  PARTIALLY_PAID         | >0 (exotic)  | partial       | Offener Betrag
  PENDING/AUTHORIZED/etc | 0            | unpaid        | (no refund rows)
  storno / offer         | 0 (forced)   | n/a           | n/a

Implementation:

  - composeInvoice.ts: after computing refundedAmount, reclassify
    paymentStatus="refunded" → "paid" when 0 < refundedAmount <
    totals.gross. requiresPayment is derived from paymentStatus, so
    it correctly stays false for partial-refund-on-paid (no GiroCode,
    no payment terms — nothing is owed).
  - i18n.ts: new `finalAmountLabel` ("Endbetrag" / "Total") in both
    languages.
  - InvoiceDocument.tsx: the final-row label now picks
    outstandingLabel vs. finalAmountLabel based on requiresPayment,
    so PARTIALLY_PAID with a refund still says "Offener Betrag"
    while PARTIALLY_REFUNDED says "Endbetrag".

Verification: render-sample now runs four refund scenarios — paid +
no refund (regression guard), full refund (status=Erstattet, final
row=Endbetrag 0,00 EUR), partial refund on a paid order (status=
Bezahlt, final row=Endbetrag, no Erstattet), and PARTIALLY_REFUNDED
with refund==gross (status stays refunded). tsc / smoke / tests /
build all green.

app/services/invoice/i18n.ts

@@ -52,6 +52,13 @@ export interface InvoiceStrings {
    *  refundedAmount`) shown when there has been a refund. "Offener
    *  Betrag" / "Outstanding amount". */
   outstandingLabel: string;
+  /** Label used in place of `outstandingLabel` when the order has been
+   *  refunded but nothing is actually owed any more (i.e. the customer
+   *  paid in full and got back only part — or all — of the gross via
+   *  refunds). "Endbetrag" / "Total". The distinction matters for
+   *  PARTIALLY_REFUNDED orders, where calling the kept portion
+   *  "outstanding" would falsely suggest the customer still owes it. */
+  finalAmountLabel: string;
   addressHeading: string;
   contactHeading: string;
   legalHeading: string;
@@ -169,6 +176,7 @@ const de: InvoiceStrings = {
   referenceLabel: "Referenz",
   refundedLabel: "Zurückerstattet",
   outstandingLabel: "Offener Betrag",
+  finalAmountLabel: "Endbetrag",
   addressHeading: "Adresse",
   contactHeading: "Kontakt",
   legalHeading: "Rechtliches",
@@ -258,6 +266,7 @@ const en: InvoiceStrings = {
   referenceLabel: "Reference",
   refundedLabel: "Refunded",
   outstandingLabel: "Outstanding amount",
+  finalAmountLabel: "Total",
   addressHeading: "Address",
   contactHeading: "Contact",
   legalHeading: "Legal",