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/product.ts

@@ -13,6 +13,9 @@ export default {
       coupon: "Coupon",
       coupons: "Available coupons",
       claim: "Claim",
+      claimed: "Claimed",
+      claimFailed: "Unable to claim this coupon",
+      loginToClaim: "Sign in to claim",
       choose: "Choose",
       selected: "Selected",
       quantity: "Quantity",
@@ -58,6 +61,9 @@ export default {
       coupon: "优惠券",
       coupons: "可用优惠券",
       claim: "领取",
+      claimed: "已领取",
+      claimFailed: "优惠券领取失败",
+      loginToClaim: "登录后领取",
       choose: "选择",
       selected: "已选",
       quantity: "数量",