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/nuxt.config.ts

@@ -9,7 +9,7 @@ export default defineNuxtConfig({
       // Domains served by the live backend; every other domain stays on the
       // fixed-data adapter. Override with NUXT_PUBLIC_LIVE_DOMAINS='["catalog"]'.
       // See openspec/changes/replace-mock-api-wave-1/design.md and waves 2-3.
-      liveDomains: ["catalog", "currency", "content", "shops", "brands", "auth", "account", "cart", "orders", "shipments", "invoices", "addresses", "coupons", "points"],
+      liveDomains: ["catalog", "currency", "content", "shops", "brands", "auth", "account", "cart", "orders", "shipments", "invoices", "addresses", "coupons", "points", "flashSales"],
       appName: "mall",
     },
   },