修复账号删除报错+添加批量删除功能

- 修复单个删除报错：删除账号前先删除关联的LoginTask（外键约束）
- 新增批量删除接口 DELETE /api/accounts/batch/delete
- 路由顺序：/batch/delete 在 /{account_id} 前注册避免路径冲突
- 前端添加批量删除按钮（带确认弹窗）
- 前端添加 batchDelete API

web/frontend/src/api/modules.ts

@@ -53,6 +53,8 @@ export const accountApi = {
     api.put<any, any>('/accounts/batch-tag', { account_ids, tag }),
   listTags: () => api.get<any, string[]>('/accounts/tags/list'),
   delete: (id: number) => api.delete<any, any>(`/accounts/${id}`),
+  batchDelete: (account_ids: number[]) =>
+    api.delete<any, any>('/accounts/batch/delete', { params: { account_ids: account_ids.join(',') } }),
 };
 
 export const loginApi = {