feat: add QR code upload to Additional Information section

[komposter]

What does this PR do?

Adds the ability to upload a payment QR code image in the Additional Information section of the invoice form. The uploaded QR code will be displayed directly below the payment totals in the generated PDF.

Changes

• Added a file input for QR codes in index.html.
• Updated InvoiceData in types.ts to include qrCode.
• Implemented localStorage persistence for the QR code in storage.ts.
• Wired up image validation and preview logic in app.ts.
• Implemented rendering logic in pdf-generator.ts to draw the QR code beside the payment totals section.
• Explicitly excluded the QR code from the shared invoice URL to avoid exceeding URL length limits, along with a warning toast message.
• Added e2e tests for QR code preview default state and share URL warning logic.

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Deploy Preview: https://69ecf7c343cd26c3af4608fd--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.

[H1D]

@komposter nice idea... But I don't like QRs being persisted into localstorage... Kinda beats whole idea of URL pre-filled form.

I'll create a similar version where you provide payment URL and QR gets generated automatically.

[H1D]

#10