feat(mall): live flash-sale page and activity price snapshots

The flash-sale page loads active sessions and their items through the shared
client with live sale price, remaining activity stock, sell-through, and a
countdown, and adds the SKU to the existing cart so checkout resolves the
authoritative price. Payment and order detail tag and render the snapshotted
activity unit price.

The flash-sale fixtures leave the page; the fixed-data adapter still serves the
domain as the rollback path.

apps/mall/pages/checkout/pay.vue

@@ -99,6 +99,7 @@ onMounted(() => void loadOrders());
               <div class="item-info">
                 <strong>{{ pick(item.product_name, locale) }}</strong>
                 <span class="muted">{{ item.sku_code }}</span>
+                <span v-if="item.flash_sale_item_id" class="activity-tag">{{ t("marketing.flashTag") }}</span>
               </div>
               <PriceText :amount-minor="item.unit_price_minor" :currency="order.currency" />
               <span class="qty">× {{ item.qty }}</span>
@@ -233,6 +234,13 @@ onMounted(() => void loadOrders());
 .discount-value {
   color: var(--mall-red);
 }
+.activity-tag {
+  align-self: flex-start;
+  padding: 1px 6px;
+  border: 1px solid var(--mall-red);
+  color: var(--mall-red);
+  font-size: 11px;
+}
 .payment-panel {
   margin-top: 16px;
   padding: 0 18px 16px;