Add payment URL QR codes to invoice PDFs

[H1D]

Summary

• Added a new Payment URL field to the invoice form and persisted it through shared URLs and reloads.
• Generated a QR code in the PDF when a payment URL is present, and adjusted the payment totals layout to make room for it.
• Made PDF generation/download flows async so QR code creation can complete before previewing or saving.
• Added Playwright coverage for payment URL defaults, persistence, sharing, reload behavior, and download flow.

Testing

• Not run in this session.
• Added/updated Playwright checks for:
  • empty default payment URL state
  • shared URL round-trip persistence
  • reload persistence
  • PDF download with a payment URL present

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Deploy Preview: https://69ed42bc9254f791859cab10--easypdf-lite.netlify.app/

Prefilled test links:

• Mixed VAT + long total row - Long item names, item notes, custom columns, and a wide sum row.
• Stripe template + pay online - Stripe layout, USD formatting, pay-online URL, and buyer notes.
• German locale + CHF formatting - Localized labels, date format, currency formatting, and custom tax label.
• Slovak locale smoke - Slovak labels, EUR formatting, and DPH tax wording.
• Shared link without QR payload - No QR data, for checking that local QR state does not leak into shared invoices.