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>

.npmrc

@@ -0,0 +1,5 @@
+# npm 12+ disables fetching git-protocol dependencies by default
+# (allow-git=none). @einfach-produktiv/invoicing is declared directly in
+# this file's own package.json (not a transitive dependency), so "root" is
+# the narrowest setting that still allows it.
+allow-git=root