Add payment URL QR codes to invoice PDFs

[H1D]

Summary

• Added a paymentUrl field to invoice data, form defaults, URL sharing, and the compressed key map so payment links persist across reloads and shared URLs.
• Generated payment QR codes in the PDF using qrcode, with layout changes to make room for the QR block and avoid overlapping the notes section.
• Updated the download and preview flow to handle async PDF generation, plus added e2e coverage for defaults, sharing, reload persistence, and PDF download behavior.
• Documented the new payment QR support in the README and updated dependency metadata.

Testing

• Added Playwright checks for an empty default payment URL, shared URL persistence, reload persistence, and download flow with a payment URL.
• Not run: unit or e2e test suite in this branch during review.

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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.