[Bug]: Coupon discount calculation ignores apply_on and always uses subtotal

[my-dev-jour]

Bug Description

POS coupons support different calculation bases such as Grand Total and Net Total, but the frontend coupon flow computes the discount from subtotal-before-tax and sends that as the final discount amount.

As a result, the effective discount can differ from the coupon's configured apply_on behavior.

Expected Behavior

Coupon discount amount should be calculated from the correct base according to the configured apply_on value.

Actual Behavior

The frontend always derives the coupon discount from subtotal instead of honoring apply_on.

Steps to Reproduce

1. Create coupons with different apply_on values, for example Grand Total and Net Total
2. Build a cart where tax and totals differ enough to expose the calculation base
3. Apply the coupon in POSNext
4. Compare the applied discount against the configured coupon basis

Impact

• wrong discount totals
• mismatch between coupon configuration and frontend behavior
• inconsistent pricing and tax outcomes

Suggested Fix

• compute discount amount from the correct pricing basis for the configured apply_on
• keep frontend and backend coupon math aligned for the final applied discount

Notes

This is a pricing-calculation bug in the POSNext frontend coupon flow, not just a display issue.