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/mock/data.ts

@@ -692,6 +692,7 @@ export function seedOrders(userId: string): MockOrderSeed {
         unit_price_minor: sku.price_minor,
         qty,
         image: product.images[0] ?? null,
+        flash_sale_item_id: null,
       };
     });
     const total = items.reduce((sum, it) => sum + it.unit_price_minor * it.qty, 0);