Move invoice/tax-breakdown PDF generation into @einfach-produktiv/invoicing

Phase 0 of the e-invoicing migration plan (see the E-Rechnung planning
session) — moves invoicePdf.tsx, correctionInvoicePdf.tsx, and
taxBreakdown.ts into a new shared package, consumed as a git dependency
by both this repo and the payload backend, instead of hand-duplicating
the correction-invoice logic between them (see that package's own README
for the three real drifts the duplication had already caused).

Consumed as raw TS/TSX source via next.config.ts's transpilePackages, not
a pre-built package. Needs `git` in the Docker deps stage and a
project .npmrc (allow-git=root) to let npm ci fetch a git-URL dependency
at all — npm 12+ disables that by default.

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

app/konto/bestellungen/[orderNumber]/page.tsx

@@ -8,7 +8,7 @@ import { VatBreakdown } from "../../../components/VatBreakdown";
 import { formatPrice, formatDate } from "../../../lib/format";
 import { getSessionCustomer, getCustomerOrderDetail, customerOrderAction } from "../../../lib/customerAuth";
 import { getProductImagesByIds } from "../../../lib/payload";
-import { computeTaxBreakdown } from "../../../lib/taxBreakdown";
+import { computeTaxBreakdown } from "@einfach-produktiv/invoicing";
 import { buildTrackingUrl, CARRIER_LABELS } from "../../../lib/tracking";
 import { OrderActionButton } from "./components/OrderActionButton";
 import { OrderStatusBadge } from "../../components/OrderStatusBadge";