Receipt Printing Support

## 🚀 Feature Request: Receipt Printing Support

### Summary
Add receipt printing functionality to BuzzPay to support hardware PoS terminals like the Bitcoinize Machine with integrated thermal printers.

### Use Case
We are using BuzzPay (pos.albylabs.com) on a **Bitcoinize Machine** – a dedicated Bitcoin PoS device with:
- 6-inch touchscreen
- Integrated 58mm thermal printer (rated for 10,000+ prints)
- Android 14 (de-Googled)
- NFC reader for Bolt Cards

BuzzPay is perfect for our needs as a minimalistic NWC-powered PoS, but we need to provide **printed receipts** to our customers after successful Lightning payments.

### Proposed Feature
After a successful payment, BuzzPay should offer an option to print a receipt containing:
- ✅ Amount (in sats and/or fiat equivalent)
- ✅ Date & Time
- ✅ Payment Preimage (shortened/QR code)
- ✅ Optional: Store name/logo
- ✅ Optional: "Powered by BuzzPay / Alby" branding

### Technical Context
The Bitcoinize Machine uses the **Print Companion** app (from GaloyMoney) for printing:
- GitHub: `GaloyMoney/pos-print-companion`
- It can receive print jobs from web apps

**Possible implementation approaches:**
1. **Android Print Intent** – If running as PWA in Android WebView
2. **Web Print API** – `window.print()` with receipt-formatted CSS
3. **Print Companion Integration** – Direct API call to the companion app
4. **ESC/POS over Web Bluetooth** – For direct printer communication

### Why This Matters
BuzzPay is ideal for onboarding merchants at markets, festivals, and in circular economies. Receipt printing is often a legal or practical requirement for merchants. This feature would make BuzzPay a complete PoS solution for hardware terminals.

### Environment
- **Device:** Bitcoinize Machine v2.1
- **OS:** Android 14 (without Google Services)
- **Printer:** Built-in 58mm thermal printer
- **BuzzPay:** pos.albylabs.com (PWA)

### References
- Bitcoinize Machine: https://bitcoinize.com
- Print Companion: https://github.com/GaloyMoney/pos-print-companion
- BuzzPay: https://pos.albylabs.com

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Thank you for building such a simple and powerful PoS! 🐝⚡
We'd be happy to help test this feature or contribute if needed.