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>

README.md

@@ -33,6 +33,8 @@ One canonical implementation, consumed by both repos, makes this class of drift
 
 **2026-07-25, unpaid-notice layout fix (v0.2.4).** The new unpaid-Vorkasse instruction (v0.2.3) reused `paidBadgeRow`, which lives inside `summary`'s `alignItems: "flex-end"` column (240pt-wide, designed for the short one-line paid confirmation) — so the longer two-sentence instruction shrank to content width and read as squeezed under the narrow summary card instead of a proper full-width note. Moved to its own sibling row (`unpaidNoticeRow`, `width: "100%"`, `marginTop: 20` vs. `paidBadgeRow`'s 10) outside `summary` entirely, left-aligned instead of right-aligned.
 
+**2026-07-28, embedded font (v0.2.6).** An E-Rechnung/PDF-A checker flagged that generated invoice PDFs don't embed their fonts — both `invoicePdf.tsx` and `correctionInvoicePdf.tsx` used react-pdf's built-in `Helvetica`/`Helvetica-Bold`, one of the "standard 14" PostScript fonts react-pdf never actually embeds (it just emits a `/BaseFont` reference and relies on the PDF viewer having a substitute installed). PDF/A-3 — already required by this package's own Factur-X/ZUGFeRD pipeline (see "E-invoicing" below) — has no exemption for standard fonts; every font actually used must be embedded. Fixed by vendoring `LiberationSans-Regular.ttf`/`LiberationSans-Bold.ttf` (SIL OFL-1.1, metrically identical to Helvetica/Arial — no layout shift) under `src/assets/fonts/`, registered once via `Font.register()` in the new `fonts.ts` (imported for its side effect by both PDF modules), with every `fontFamily: "Helvetica"`/`"Helvetica-Bold"` replaced by `fontFamily: "Liberation Sans"` (+ `fontWeight: "bold"` where the bold variant was used). Verified by inspecting the raw PDF bytes of a generated fixture: `/FontFile2` present, `/BaseFont` shows subset tags (e.g. `CWOPQE+LiberationSans`), `/Subtype /Type0` — i.e. actually embedded, not just referenced.
+
 ## How this is consumed
 
 Not published to npm — installed as a git dependency:
@@ -51,6 +53,7 @@ Ships raw TypeScript/TSX source (no build step) via `main`/`types` pointing stra
 - `formatters.ts` — `formatPrice()`/`formatDate()`, canonical formatting for every document.
 - `invoicePdf.tsx` — the original invoice ("Rechnung"): `InvoiceDocument`, `renderInvoicePdf()`, plus `SAMPLE_INVOICE_ORDER` (used by the frontend's Payload Live Preview for company-settings).
 - `correctionInvoicePdf.tsx` — Stornorechnung/Gutschrift: `renderCorrectionInvoicePdf()`.
+- `fonts.ts` — registers the embedded Liberation Sans font (`src/assets/fonts/`) once for both PDF modules — see the 2026-07-28 changelog entry above.
 - `seller.ts` — the shared `InvoiceSeller` type both document types render in their footer.
 - `einvoice/` — the ZUGFeRD/Factur-X layer (see "E-invoicing" below).