fix: honor coupon apply_on in POS

[my-dev-jour]

Summary

• honor coupon apply_on when computing the discount amount in the POS coupon dialog
• use the same selected base amount when applying the coupon to the cart so percentage coupons are not recalculated against subtotal
• pass tax and grand total into the coupon dialog so Grand Total and Net Total coupons produce different results as intended

Root cause

CouponDialog.vue always validated min_amount and calculated the coupon from subtotal, even when the coupon itself carried apply_on = "Grand Total" or apply_on = "Net Total".

The stale frontend calculation was then repeated in useInvoice.applyDiscount(), which recalculated percentage discounts against subtotal again.

Validation

• confirmed on the staging code path before patching: coupon dialog ignored coupon.apply_on and always used subtotal
• confirmed current upstream code still had the same behavior before patching
• deployed the patch to a staging environment and rebuilt POS assets
• local verification was limited to git diff --check in this workspace