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/locales/marketing.ts

@@ -11,6 +11,8 @@ export default {
       originalPrice: "Original price",
       progress: "Sold {n}%",
       buyNow: "Grab now",
+      grabFailed: "Unable to add this item right now.",
+      flashTag: "Flash sale",
       noSeckillProducts: "No flash-sale products right now.",
       collectiveTitle: "Group deals",
       collectiveBreadcrumb: "Group deals",
@@ -69,6 +71,8 @@ export default {
       originalPrice: "原价",
       progress: "已抢 {n}%",
       buyNow: "马上抢",
+      grabFailed: "加入购物车失败，请稍后重试。",
+      flashTag: "秒杀价",
       noSeckillProducts: "当前暂无秒杀商品。",
       collectiveTitle: "拼团购",
       collectiveBreadcrumb: "拼团",