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>

src/fonts.ts

@@ -0,0 +1,21 @@
+import { Font } from "@react-pdf/renderer";
+
+// Registered once here (not inline in invoicePdf.tsx/correctionInvoicePdf.tsx)
+// and imported by both for its side effect — avoids double-registering the
+// same family when renderEInvoice.ts imports both PDF modules in the same
+// process.
+//
+// Liberation Sans, not built-in Helvetica: PDF/A-3 (required by the
+// Factur-X/ZUGFeRD pipeline in src/einvoice/) has no exemption for the
+// standard/base-14 fonts react-pdf ships — every font actually used must be
+// embedded, and react-pdf only embeds fonts it has real font-file bytes for
+// via Font.register. Liberation Sans is metrically identical to Helvetica/
+// Arial (SIL Open Font License, see src/assets/fonts/LICENSE-OFL.txt), so
+// embedding it doesn't shift any existing layout.
+Font.register({
+  family: "Liberation Sans",
+  fonts: [
+    { src: new URL("./assets/fonts/LiberationSans-Regular.ttf", import.meta.url).pathname, fontWeight: "normal" },
+    { src: new URL("./assets/fonts/LiberationSans-Bold.ttf", import.meta.url).pathname, fontWeight: "bold" },
+  ],
+});