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/pages/checkout/pay.vue

@@ -107,6 +107,12 @@ onMounted(() => void loadOrders());
               </strong>
             </div>
             <footer class="order-total">
+              <template v-if="order.discount_minor > 0">
+                <span>{{ t("checkout.couponDiscount") }}</span>
+                <span class="discount-value">
+                  −<PriceText :amount-minor="order.discount_minor" :currency="order.currency" />
+                </span>
+              </template>
               <span>{{ t("checkout.total") }}</span>
               <strong><PriceText :amount-minor="order.total_minor" :currency="order.currency" /></strong>
             </footer>
@@ -224,6 +230,9 @@ onMounted(() => void loadOrders());
   color: var(--mall-red);
   font-size: 16px;
 }
+.discount-value {
+  color: var(--mall-red);
+}
 .payment-panel {
   margin-top: 16px;
   padding: 0 18px 16px;