feat: 社保公积金账户化重构

新增 SocialAccount（账户）+ SocialYearStandard（年度标准）两层实体，
替代原 SocialInsuranceConfig/HousingFundConfig 按城市管理的方式。

- DB: 新增 SocialAccount、SocialYearStandard 表，Department 加账户关联
- 迁移: 旧 Config 表数据迁移到 Account + YearStandard
- 后端: 新增账户 CRUD + 年度标准 API，薪资计算适配 accountId
- 前端: 设置页新增账户管理 Tab，组织架构提示 level=0 可关联账户
- 前端: SocialInsurance.tsx 城市选择器改为账户选择器
- 兼容: 旧 Config 表保留，薪资计算回退旧表

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

frontend/src/lib/api-services.ts

@@ -515,6 +515,40 @@ export const salaryDashboardApi = {
 
 // ========== 社保公积金相关 ==========
 
+// 账户管理（新）
+export const socialAccountApi = {
+  /** 账户列表 */
+  list: (type?: string) =>
+    get('/social/accounts', { params: type ? { type } : {} }).then(unwrap<any[]>()),
+  /** 新建账户 */
+  create: (data: { type: string; name: string; city: string; accountNo?: string; bankName?: string; bankAccount?: string; orgName?: string; orgCode?: string; accountType?: string; isDefault?: boolean; remark?: string }) =>
+    post('/social/accounts', data).then(unwrap<any>()),
+  /** 编辑账户 */
+  update: (id: string, data: Partial<{ type: string; name: string; city: string; accountNo: string; bankName: string; bankAccount: string; orgName: string; orgCode: string; accountType: string; isDefault: boolean; remark: string; status: string }>) =>
+    put(`/social/accounts/${id}`, data).then(unwrap<any>()),
+  /** 删除账户 */
+  remove: (id: string) =>
+    del(`/social/accounts/${id}`).then(unwrap<any>()),
+  /** 设为默认 */
+  setDefault: (id: string) =>
+    put(`/social/accounts/${id}/default`).then(unwrap<any>()),
+  /** 按账户获取年度标准列表 */
+  standards: (accountId: string) =>
+    get(`/social/accounts/${accountId}/standards`).then(unwrap<any[]>()),
+  /** 按账户获取当前生效标准 */
+  currentStandard: (accountId: string) =>
+    get(`/social/accounts/${accountId}/current-standard`).then(unwrap<any>()),
+  /** 按账户+月份获取适用标准 */
+  standardByMonth: (accountId: string, month: string) =>
+    get(`/social/accounts/${accountId}/standard-by-month/${month}`).then(unwrap<any>()),
+  /** 新建年度标准 */
+  createStandard: (accountId: string, data: any) =>
+    post(`/social/accounts/${accountId}/standards`, data).then(unwrap<any>()),
+  /** 按员工获取适用账户 */
+  employeeAccount: (employeeId: string) =>
+    get(`/social/employee-account/${employeeId}`).then(unwrap<any>()),
+}
+
 export const socialInsuranceApi = {
   /** 城市列表 */
   cities: () =>