feat(mall): claim and redeem shop coupons from the storefront

The product page loads a shop's claimable coupons through the shared client and
claims them in place; the buyer coupon list reads owned snapshots with their
claimed/redeemed/expired state. Checkout offers each shop order at most one
owned coupon and submits only the choice; the pay page and order detail render
the server-persisted discount and reduced total, never a client figure.

Coupon fixtures leave the pages; the fixed-data adapter still serves the coupon
domain as the rollback path. The demo seed creates two deterministic templates
and claims them for the demo customer.

apps/mall/locales/checkout.ts

@@ -23,6 +23,9 @@ export default {
       alipay: "Alipay",
       payTitle: "Pay for your order",
       orderNo: "Order no.",
+      coupon: "Coupon",
+      noCoupon: "No coupon",
+      couponDiscount: "Coupon discount",
       grandTotal: "Total payment",
       confirmPay: "Confirm payment",
       noPendingOrders: "There are no orders waiting for payment.",
@@ -58,6 +61,9 @@ export default {
       alipay: "支付宝",
       payTitle: "订单支付",
       orderNo: "订单号",
+      coupon: "优惠券",
+      noCoupon: "不使用优惠券",
+      couponDiscount: "优惠券抵扣",
       grandTotal: "应付总额",
       confirmPay: "确认支付",
       noPendingOrders: "暂无待支付订单。",