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/api.ts

@@ -15,6 +15,10 @@ import type {
   Coupon,
   CouponTemplate,
   CouponTemplateInput,
+  FlashSaleItem,
+  FlashSaleItemInput,
+  FlashSaleSession,
+  FlashSaleSessionInput,
   HomeContent,
   IntegralOrder,
   IntegralProduct,
@@ -23,6 +27,7 @@ import type {
   InvoiceKind,
   Order,
   Product,
+  PublicFlashSaleSession,
   RedeemPointsBody,
   Shipment,
   ShopProfile,
@@ -43,9 +48,11 @@ import {
   USER_STATS,
   MOCK_ADDRESSES,
   defaultAddress,
+  lowestSku,
   mockConvertMinor,
   productById,
   searchMockProducts,
+  seckillProducts,
   seedOrders,
   storeById,
 } from "./data";
@@ -150,6 +157,46 @@ function seedPointsProducts(): IntegralProduct[] {
   }));
 }
 
+/** One synthesized active session covering the fixed seckill fixtures. */
+function mockFlashSales(): PublicFlashSaleSession[] {
+  const now = Date.now();
+  const items = seckillProducts().map((entry, index) => {
+    const sku = lowestSku(entry.product);
+    const stock = sku?.stock ?? 100;
+    const sold = Math.round((stock * entry.soldPct) / 100);
+    return {
+      id: `fsi${index + 1}`,
+      session_id: "fs1",
+      sku_id: sku?.id ?? "",
+      product_id: entry.product.id,
+      product_slug: entry.product.slug,
+      product_name: entry.product.name,
+      image: entry.product.images[0] ?? null,
+      sku_code: sku?.sku_code ?? "",
+      sale_price_minor: entry.seckillPriceMinor,
+      currency: sku?.currency ?? BASE_CURRENCY,
+      original_price_minor: sku?.price_minor ?? entry.seckillPriceMinor,
+      original_currency: sku?.currency ?? BASE_CURRENCY,
+      reserved_stock: Math.max(stock - sold, 0),
+      sold_count: sold,
+      per_customer_limit: 2,
+    };
+  });
+  return [
+    {
+      id: "fs1",
+      shop_id: MOCK_STORES[0].id,
+      label: { en: "Flash sale", zh: "限时秒杀" },
+      starts_at: new Date(now - 3_600_000).toISOString(),
+      ends_at: new Date(now + 6 * 3_600_000).toISOString(),
+      enabled: true,
+      created_at: "2026-01-01T00:00:00.000Z",
+      updated_at: "2026-01-01T00:00:00.000Z",
+      items,
+    },
+  ];
+}
+
 function initialState(): MockState {
   const persisted = loadPersisted();
   // Coupons and points are session-only, so a restored snapshot re-seeds them.
@@ -397,6 +444,7 @@ export function createMockApi(): ApiClient {
             unit_price_minor: i.unit_price_minor,
             qty: i.qty,
             image: i.image,
+            flash_sale_item_id: null,
           })),
           shipping_address: shippingAddress,
           created_at: new Date().toISOString(),
@@ -630,6 +678,8 @@ export function createMockApi(): ApiClient {
         state.pointsProducts.filter((p) => p.published).map((p) => ({ ...p })),
       ),
 
+    listFlashSales: () => Promise.resolve(mockFlashSales()),
+
     listMyRedemptions: () => Promise.resolve(state.redemptions.map((o) => ({ ...o }))),
 
     redeemPoints: (body: RedeemPointsBody) => {
@@ -693,6 +743,13 @@ export function createMockApi(): ApiClient {
       createCouponTemplate: (_body: CouponTemplateInput) => unsupported(),
       updateCouponTemplate: (_id: string, _body: CouponTemplateInput) => unsupported(),
       deleteCouponTemplate: (_id: string) => unsupported(),
+      listFlashSales: () => unsupported(),
+      createFlashSale: (_body: FlashSaleSessionInput) => unsupported(),
+      updateFlashSale: (_id: string, _body: FlashSaleSessionInput) => unsupported(),
+      deleteFlashSale: (_id: string) => unsupported(),
+      addFlashSaleItem: (_sessionId: string, _body: FlashSaleItemInput) => unsupported(),
+      updateFlashSaleItem: (_id: string, _body: FlashSaleItemInput) => unsupported(),
+      deleteFlashSaleItem: (_id: string) => unsupported(),
     },
     admin: {
       listUsers: () => unsupported(),