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/user.ts

@@ -83,6 +83,10 @@ export default {
       threshold: "Minimum spend",
       expiresAt: "Expires",
       noCoupons: "No coupons yet.",
+      couponStatus: "Status",
+      couponClaimed: "Claimed",
+      couponRedeemed: "Used",
+      couponExpired: "Expired",
       invoicesTitle: "My invoices",
       invoiceNo: "Invoice no.",
       invoiceOrderNo: "Order no.",
@@ -176,6 +180,10 @@ export default {
       threshold: "使用门槛",
       expiresAt: "有效期至",
       noCoupons: "暂无优惠券。",
+      couponStatus: "状态",
+      couponClaimed: "未使用",
+      couponRedeemed: "已使用",
+      couponExpired: "已过期",
       invoicesTitle: "我的发票",
       invoiceNo: "发票号",
       invoiceOrderNo: "订单号",