feat(pos-selfpay-flow): pOS selfpay: simplified pay flow with optional email receipt

work/artifacts/POS-SELFPAY-FLOW/implementer.md

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+# Implementer — POS-SELFPAY-FLOW
+
+## Summary
+Simplified selfpay flow: tap Pagar → optional email modal → payment methods → sale.
+
+## Changes
+
+### States added
+- `showSelfpayEmail` — controls the email modal visibility
+- `selfpayEmail` — stores the optional email for the receipt
+
+### Selfpay button
+- In selfpay mode, "Cobrar e imprimir" is replaced by "💳 Pagar"
+- Tapping "Pagar" shows the email modal
+- The button is always enabled in selfpay mode (no payment requirement to start)
+
+### Payment methods
+- In selfpay mode, payment methods are hidden until after the email modal is shown
+- After email (or skip), payment methods appear and normal flow continues
+
+### Email modal
+- Optional email input for receipt
+- "Continuar al pago" submits and closes the modal
+- "Omitir" closes without email
+
+## Files
+- `app/(terminal)/page.tsx`
+
+## Validation
+- `cd project/apps/pos && npm run build` ✅
+- `git diff --check` ✅