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/index.vue

@@ -1,6 +1,6 @@
 <script setup lang="ts">
 import { t as pick } from "@vmall/shared";
-import type { Address, AddressBookEntry, CartItem, LocalizedText } from "@vmall/shared";
+import type { Address, AddressBookEntry, CartItem, Coupon, LocalizedText } from "@vmall/shared";
 
 type CheckoutGroup = {
   shopId: string;
@@ -19,6 +19,9 @@ const pendingOrderIds = useState<string[]>("checkout-orders", () => []);
 
 const items = ref<CartItem[]>([]);
 const addresses = ref<AddressBookEntry[]>([]);
+/** Owned, still-claimable coupons, by shop. */
+const coupons = ref<Coupon[]>([]);
+const selectedCoupon = reactive<Record<string, string>>({});
 const selectedAddressId = ref("");
 const remark = ref("");
 const loading = ref(true);
@@ -67,6 +70,17 @@ function imageFor(item: CartItem): string {
   return item.image ?? "/mock/product-1.svg";
 }
 
+/** One coupon per shop order; the server decides eligibility and the amount. */
+function couponsForShop(shopId: string): Coupon[] {
+  return coupons.value.filter((c) => c.shop_id === shopId && c.status === "claimed");
+}
+
+function selectedCouponFor(shopId: string): Coupon | null {
+  const id = selectedCoupon[shopId];
+  if (!id) return null;
+  return coupons.value.find((c) => c.id === id) ?? null;
+}
+
 function selectedAddress(): Address | null {
   const saved = addresses.value.find((a) => a.id === selectedAddressId.value);
   if (saved) {
@@ -92,9 +106,15 @@ async function loadCart(): Promise<void> {
   loading.value = true;
   error.value = "";
   try {
-    const [snapshot, list] = await Promise.all([$api.getCart(), $api.listMyAddresses()]);
+    const [snapshot, list, couponList] = await Promise.all([
+      $api.getCart(),
+      $api.listMyAddresses(),
+      // Coupons are optional: a failure must not block checkout.
+      $api.listMyCoupons().catch(() => [] as Coupon[]),
+    ]);
     items.value = snapshot.items;
     addresses.value = list;
+    coupons.value = couponList;
     selectedAddressId.value =
       list.find((address) => address.is_default)?.id ?? list[0]?.id ?? "";
     if (items.value.length === 0) {
@@ -116,7 +136,11 @@ async function submitOrder(): Promise<void> {
   submitting.value = true;
   error.value = "";
   try {
-    const orders = await $api.checkout(address, currency.value);
+    const couponByShop: Record<string, string> = {};
+    for (const [shopId, couponId] of Object.entries(selectedCoupon)) {
+      if (couponId) couponByShop[shopId] = couponId;
+    }
+    const orders = await $api.checkout(address, currency.value, couponByShop);
     pendingOrderIds.value = orders.map((order) => order.id);
     const snapshot = await $api.getCart();
     items.value = snapshot.items;
@@ -192,6 +216,22 @@ onMounted(() => void loadCart());
                 <PriceText :amount-minor="item.unit_price_minor * item.qty" :currency="item.currency" />
               </strong>
             </div>
+            <div v-if="couponsForShop(group.shopId).length > 0" class="coupon-picker">
+              <span class="coupon-label">{{ t("checkout.coupon") }}</span>
+              <select v-model="selectedCoupon[group.shopId]" class="minput coupon-select">
+                <option value="">{{ t("checkout.noCoupon") }}</option>
+                <option v-for="coupon in couponsForShop(group.shopId)" :key="coupon.id" :value="coupon.id">
+                  {{ pick(coupon.title, locale) }}
+                </option>
+              </select>
+              <span v-if="selectedCouponFor(group.shopId)" class="coupon-value">
+                −
+                <PriceText
+                  :amount-minor="selectedCouponFor(group.shopId)?.amount_minor ?? 0"
+                  :currency="selectedCouponFor(group.shopId)?.currency ?? sourceCurrency"
+                />
+              </span>
+            </div>
           </div>
         </section>
 
@@ -348,10 +388,27 @@ onMounted(() => void loadCart());
   color: var(--mall-red);
   text-align: right;
 }
+.coupon-picker {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 10px 18px 14px;
+  border-top: 1px solid #f5f5f5;
+}
+.coupon-label {
+  color: var(--mall-muted);
+  font-size: 13px;
+}
+.coupon-select {
+  max-width: 320px;
+}
+.coupon-value {
+  color: var(--mall-red);
+  font-size: 13px;
+}
 .remark-panel {
   padding: 0 0 16px;
-}
-.remark-input {
+}.remark-input {
   display: block;
   box-sizing: border-box;
   width: calc(100% - 36px);