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/user/orders/[id].vue

@@ -118,6 +118,10 @@ onMounted(() => {
             </tr>
           </tbody>
         </table>
+        <p v-if="order.discount_minor > 0" class="order-discount">
+          {{ t("checkout.couponDiscount") }}:
+          <PriceText :amount-minor="order.discount_minor" :currency="order.currency" />
+        </p>
         <p class="order-total">{{ t("user.total") }}: <PriceText :amount-minor="order.total_minor" :currency="order.currency" /></p>
       </article>
 
@@ -218,6 +222,12 @@ onMounted(() => {
   border: 1px solid var(--mall-line);
   object-fit: contain;
 }
+.order-discount {
+  margin: 16px 0 0;
+  text-align: right;
+  font-size: 13px;
+  color: var(--mall-red);
+}
 .order-total {
   margin: 16px 0 0;
   text-align: right;