[PROMO-009] Promotions: Coupon code entry at checkout

## Overview
Add a coupon code input to the checkout flow. When a valid code is entered, the discount is applied to the cart in real time.

## Acceptance Criteria
- [ ] Coupon input field in the cart drawer and on the checkout page
- [ ] `POST /api/promotions/coupon/validate` — validates code and returns discount details
- [ ] Applied discount line shown in cart summary (e.g., "SAVE20 — 20% off: -$24.00")
- [ ] Only one coupon can be active at a time (clear button to remove)
- [ ] Expired, invalid, and already-used codes show specific error messages
- [ ] Coupon codes are case-insensitive
- [ ] Coupon application survives page refresh (stored in cart state)

## ⚠️ Status: Blocked
Blocked by both #11 (coupon codes are a discount rule type) and #3 (cart API needed for server-side coupon validation).

## Depends On
- #11 PROMO-007 — coupon codes are implemented as promotion rules
- #3 CART-003 — cart API applies the coupon server-side