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/cart/components/CartContent.tsx

@@ -8,7 +8,7 @@ import { useCart, removeFromCart, setQuantity } from "../../lib/cart";
 import { useProducts } from "../../lib/products";
 import { useDiscount, applyDiscount, clearDiscount } from "../../lib/discount";
 import { computeSubtotal, computeCartTotals, effectivePrice, effectiveTaxRate } from "../../lib/cartTotals";
-import { computeTaxBreakdown } from "../../lib/taxBreakdown";
+import { computeTaxBreakdown } from "@einfach-produktiv/invoicing";
 import { formatPrice, discountPercent } from "../../lib/format";
 import { Reveal } from "../../components/Reveal";
 import { VersandModal } from "../../components/VersandModal";