feat: add invoice PDF export

[newmattock]

Fixes #21

Summary

• add a client-side Download PDF button to the invoice detail/settlement page
• generate ILN-branded invoice PDFs with invoice ID, submitter, payer, amount, token, discount rate, due date, current status, invoice URL, and QR code
• save PDFs with the required ILN-Invoice-[ID].pdf filename
• add PDF serialization helpers plus focused coverage for helper rows, PDF generation, and the pay invoice page button
• include narrow current-main build fixes for governance Vetoed status/deferred refresh and typed federation home-domain access

Validation

• npm test -- src/utils/tests/invoicePdf.test.ts src/components/tests/InvoicePdfDownloadButton.test.tsx app/pay/[id]/tests/PayInvoice.test.tsx --reporter verbose
• npx eslint src/utils/invoicePdf.ts src/utils/tests/invoicePdf.test.ts src/components/InvoicePdfDownloadButton.tsx src/components/tests/InvoicePdfDownloadButton.test.tsx app/pay/[id]/page.tsx app/pay/[id]/tests/PayInvoice.test.tsx app/governance/page.tsx src/utils/federation.ts vitest.config.ts
• npm run build
• git diff --check
• local smoke: HEAD /pay/1 returned HTTP 200 on port 3020
• in-app browser smoke: /pay/1 exposed invoice page content and Download PDF buttons