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/user/orders/[id].vue

@@ -111,6 +111,7 @@ onMounted(() => {
               <td class="prod-col">
                 <img :src="item.image || '/mock/product-1.svg'" :alt="pick(item.product_name, locale)" />
                 <span>{{ pick(item.product_name, locale) }}</span>
+                <span v-if="item.flash_sale_item_id" class="activity-tag">{{ t("marketing.flashTag") }}</span>
               </td>
               <td><PriceText :amount-minor="item.unit_price_minor" :currency="order.currency" /></td>
               <td>{{ item.qty }}</td>
@@ -228,6 +229,13 @@ onMounted(() => {
   font-size: 13px;
   color: var(--mall-red);
 }
+.activity-tag {
+  margin-left: 6px;
+  padding: 1px 6px;
+  border: 1px solid var(--mall-red);
+  color: var(--mall-red);
+  font-size: 11px;
+}
 .order-total {
   margin: 16px 0 0;
   text-align: right;