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>

package.json

@@ -10,6 +10,7 @@
     "test:unit": "vitest run"
   },
   "dependencies": {
+    "@einfach-produktiv/invoicing": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#main",
     "@payloadcms/live-preview-react": "^3.85.2",
     "@react-pdf/renderer": "^4.5.1",
     "motion": "^12.42.2",