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/company-settings-preview/components/LiveCompanySettingsPreviewClient.tsx

@@ -2,7 +2,7 @@
 
 import dynamic from "next/dynamic";
 import { useLivePreview } from "@payloadcms/live-preview-react";
-import { InvoiceDocument, SAMPLE_INVOICE_ORDER } from "../../lib/invoicePdf";
+import { InvoiceDocument, SAMPLE_INVOICE_ORDER } from "@einfach-produktiv/invoicing";
 import type { CompanySettings } from "../../lib/payload";
 
 const PAYLOAD_URL = process.env.NEXT_PUBLIC_PAYLOAD_URL || "https://payload.mk360.de";