Add Module 3: Receipt Printing

[ZuhaibAkhtarKhan]

Summary

• Implements Module 3 receipt printing: light Cash/Card payment UI, thermal receipt preview, and browser print flow for 58mm/80mm printers
• Adds receipts API (GET /api/v1/receipts, GET /api/v1/receipts/:id, PATCH /api/v1/receipts/:id/print) plus payment_method / printed_at on orders
• Includes print helper for Module 2 checkout integration and backend receipt tests

Test plan

• Start backend (npm run dev in backend) and frontend (npm run dev in frontend)
• Open /receipts and confirm the white payment + receipt preview UI loads
• Select Cash/Card, pick an order, click Pay & Print and verify the print dialog opens
• Run npm test in backend and confirm receipt API tests pass
• Run npm run lint and npm run build in frontend

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