feat: implement local PDF receipt generation for buyers

[OperaCode]

Summary

• add a buyer-facing Download Receipt action on the checkout success state
• generate the receipt PDF locally in the browser using existing checkout payment data
• include key receipt fields such as amount, asset, date, payment ID, recipient, and transaction hash
• add checkout copy updates and Playwright coverage for the new receipt action

Closes #427

Buyers need a simple way to save proof of payment after a successful checkout without introducing server-side PDF generation or extra backend load.

What changed

• added a Download Receipt button to the settled payment UI on the public checkout page
• created a lightweight client-side PDF generator for receipt downloads
• used the checkout’s existing settled payment data to build the receipt
• added localized strings for receipt download states
• added e2e tests to verify the receipt action is visible and triggers a PDF download

Validation

• attempted npm run lint but it is currently blocked by an existing repo dependency issue: missing next-intl/plugin
• attempted npx playwright test tests/e2e/checkout.spec.ts --grep "receipt|settled" but it is blocked by the same missing dependency during web server startup

Notes

• this implementation keeps PDF generation fully client-side to match the issue requirements and avoid backend load

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