feat(pos): add point of sale UI

- Add ProductGrid component with category filters and search
- Add Cart component with quantity controls and totals
- Add PaymentDialog with cash/card/transfer payment methods
- Add CashRegisterStatus for opening/closing register
- Add POS page with 70/30 layout (products/cart)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/components/pos/index.ts

@@ -0,0 +1,5 @@
+export { ProductGrid } from "./product-grid";
+export { Cart } from "./cart";
+export type { CartItem } from "./cart";
+export { PaymentDialog } from "./payment-dialog";
+export { CashRegisterStatus } from "./cash-register-status";