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>

Dockerfile

@@ -1,9 +1,11 @@
 FROM node:20-alpine AS base
 
 FROM base AS deps
-RUN apk add --no-cache libc6-compat
+# git — needed for `npm ci` to fetch @einfach-produktiv/invoicing, a git-URL
+# dependency (see package.json); Alpine's base image doesn't ship it.
+RUN apk add --no-cache libc6-compat git
 WORKDIR /app
-COPY package*.json ./
+COPY package*.json .npmrc ./
 RUN npm ci
 
 FROM base AS builder