feat(admin): 平台商户成员管理改用抽屉，消除商户视角切换与全局状态污染

- 交互重构：在「平台商户」与「平台账号」点击成员管理时，不再跳转路由或改写 localStorage.merchant_id，改为滑出 MerchantMembersDrawer 抽屉
- 抽屉能力：支持查看指定商户成员列表、新增成员（新建账号或关联已有账号）、修改角色与启用状态、移除成员
- 顶栏清理：彻底移除顶栏右上角的「当前商户：xxx」标签与「退出商户视角」按钮，超管登录自动清理任何残留上下文
- 后端补齐：新增平台专属 /api/platform/merchants/:id/members 与 roles 接口，平台操作不再依赖 X-Merchant-ID
- 验证：tsc -b && vite build 成功；全量 go test ./... 包含路由与鉴权回归测试 100% 通过

frontend/src/api/index.ts

@@ -230,10 +230,21 @@ export const platformApi = {
   }) => request.post('/platform/merchants', data).then((r) => r.data.data as Merchant),
   updateMerchant: (id: number, data: Partial<Merchant>) =>
     request.patch(`/platform/merchants/${id}`, data).then((r) => r.data.data),
+  members: (merchantId: number) =>
+    request.get(`/platform/merchants/${merchantId}/members`).then((r) => r.data.data as MerchantMember[]),
   addMember: (
     merchantId: number,
-	data: { user_id?: number; username?: string; password?: string; nickname?: string; role: string; is_default?: boolean },
+    data: { user_id?: number; username?: string; password?: string; nickname?: string; role: string; is_default?: boolean },
   ) => request.post(`/platform/merchants/${merchantId}/members`, data).then((r) => r.data.data as MerchantMember),
+  updateMember: (
+    merchantId: number,
+    memberId: number,
+    data: { role: string; status: number; is_default: boolean },
+  ) => request.patch(`/platform/merchants/${merchantId}/members/${memberId}`, data).then((r) => r.data.data as MerchantMember),
+  removeMember: (merchantId: number, memberId: number) =>
+    request.delete(`/platform/merchants/${merchantId}/members/${memberId}`).then((r) => r.data.data),
+  roles: (merchantId: number) =>
+    request.get(`/platform/merchants/${merchantId}/roles`).then((r) => r.data.data as MerchantRole[]),
   productCatalog: () =>
     request.get('/platform/product-catalog').then((r) => r.data.data as ProductCatalogItem[]),
   merchantProducts: (merchantId: number) =>