Embed Liberation Sans in invoice PDFs for PDF/A-3 compliance (v0.2.6)

An E-Rechnung checker flagged that generated invoices don't embed their
fonts. Both invoicePdf.tsx and correctionInvoicePdf.tsx used react-pdf's
built-in Helvetica/Helvetica-Bold, which is never embedded — PDF/A-3
(already required by this package's own Factur-X/ZUGFeRD pipeline) has
no exemption for standard fonts. Vendors Liberation Sans (SIL OFL-1.1,
metrically identical to Helvetica) and registers it via Font.register in
a new fonts.ts, imported by both PDF modules.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.5",
+  "version": "0.2.6",
   "private": true,
   "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
   "type": "module",