feat(admin): 品牌管理 + 新建商品

品牌管理：列出品牌及引用商品数，支持改名、合并重复品牌（把源品牌的商品并入目标后删除源）、删除未被引用的品牌。新建商品：商品列表新增「新建商品」入口，填写名称/条码/品牌/品类后创建并进入详情页继续补全。

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

admin-frontend/src/api.ts

@@ -61,6 +61,11 @@ export const api = {
     }>(`/products?q=${encodeURIComponent(q)}&page=${page}&size=${size}`),
   getProduct: (id: string) =>
     request<import("./types").ProductDetail>(`/products/${id}`),
+  createProduct: (body: unknown) =>
+    request<import("./types").ProductDetail>("/products", {
+      method: "POST",
+      body: JSON.stringify(body),
+    }),
   updateProduct: (id: string, body: unknown) =>
     request<import("./types").ProductDetail>(`/products/${id}`, {
       method: "PUT",
@@ -102,6 +107,23 @@ export const api = {
     ),
   listBrands: () =>
     request<{ items: import("./types").Brand[] }>("/brands"),
+  createBrand: (name: string) =>
+    request<import("./types").Brand>("/brands", {
+      method: "POST",
+      body: JSON.stringify({ name }),
+    }),
+  updateBrand: (id: string, name: string) =>
+    request<import("./types").Brand>(`/brands/${id}`, {
+      method: "PUT",
+      body: JSON.stringify({ name }),
+    }),
+  mergeBrands: (id: string, targetId: string) =>
+    request<import("./types").Brand>(`/brands/${id}/merge`, {
+      method: "POST",
+      body: JSON.stringify({ target_id: targetId }),
+    }),
+  deleteBrand: (id: string) =>
+    request<{ status: string }>(`/brands/${id}`, { method: "DELETE" }),
   listCategories: () =>
     request<{ items: import("./types").Category[] }>("/categories"),
   createCategory: (body: import("./types").CategoryInput) =>