User Story: Move discount validation server-side for cart discounts in src/javascript/frontend/services/cart.js

[aliAljaffer]

As a developer
I want to avoid relying on client-side discount validation in src/javascript/frontend/services/cart.js
So that users cannot tamper with discounts and apply unauthorized reductions

Acceptance Criteria

• Remove or minimize business-critical discount validation on the client. Ensure the server verifies discount codes and calculates final totals.

Details
The file uses a local discounts map and computes discount amounts client-side (comment: "Client-side discount validation (insecure)"). This allows malicious users to bypass rules.