[Invoices]: Include client reference & work order fields in PDF template output

[nielsdrost7]

client_reference and work_order (added in PR #585) are not yet rendered anywhere in the invoice/quote PDF templates. Add them to the default PDF template(s) — conditionally, only when non-empty — so the values are visible to the client, not just internally.

Related PDF infrastructure: the invoices::pdf.invoice Blade template and InvoiceService::renderHtml()/generatePdf() introduced in PR #575.

Acceptance criteria

• Generated invoice/quote PDFs show client reference and work order near the document header/details when set.
• Omitted cleanly when null.