feat: 平台管理员可在前端分配商品给商户

后端:
- service: 新增 ListProductCatalog(自营商户商品目录)、ListMerchantProductsByAdmin、AssignProducts(按目录ID批量同步商户商品，未勾选的已有商品会被移除)
- handler + router: 新增 GET /platform/product-catalog、GET /platform/merchants/:id/products、POST /platform/merchants/:id/products/assign 三个管理员路由

前端:
- types: 新增 ProductCatalogItem 类型
- api: platformApi 新增 productCatalog / merchantProducts / assignProducts
- PlatformMerchants: 商户列表操作列新增「分配商品」按钮，弹窗内以表格+复选框展示平台商品目录，支持全选/清空/重置，打开时按已有商品预选，保存后同步分配结果

实测：创建商户自动获得27个默认商品，管理员可随时增减调整

frontend/src/api/index.ts

@@ -11,6 +11,7 @@ import type {
   MerchantMember,
   MerchantProduct,
   PageResult,
+  ProductCatalogItem,
   User,
   WalletAccount,
   WalletLedgerEntry,
@@ -112,4 +113,10 @@ export const platformApi = {
     merchantId: number,
     data: { user_id: number; role: MerchantMember['role']; is_default?: boolean },
   ) => request.post(`/platform/merchants/${merchantId}/members`, data).then((r) => r.data.data as MerchantMember),
+  productCatalog: () =>
+    request.get('/platform/product-catalog').then((r) => r.data.data as ProductCatalogItem[]),
+  merchantProducts: (merchantId: number) =>
+    request.get(`/platform/merchants/${merchantId}/products`).then((r) => r.data.data as ProductCatalogItem[]),
+  assignProducts: (merchantId: number, catalogIds: number[]) =>
+    request.post(`/platform/merchants/${merchantId}/products/assign`, { catalog_ids: catalogIds }).then((r) => r.data.data as { assigned: number }),
 }