feat: 商业保险/员工福利独立页面+易签宝电子签署框架

- 商业保险：从社公商保中拆出为独立页面，侧边栏新增「福利保障」分组
- 员工福利：新建完整模块（方案管理+批量参保+员工汇总），Prisma模型+后端路由+前端页面
- 电子签署：搭建易签宝对接框架（ESignRecord模型+创建/查询/取消/回调接口+前端签署管理页面）
- 侧边栏新增「福利保障」分组：商业保险、员工福利、电子签署
- Prisma schema 新增6个模型：CommercialInsurancePlan/Enrollment, EmployeeBenefitPlan/Enrollment, ESignRecord

frontend/src/components/layout/SidebarNav.tsx

@@ -15,6 +15,7 @@ import {
   Bell, ScrollText, Settings,
   ChevronDown, ChevronRight,
   Building2, CalendarDays, ClipboardList, Heart, CalendarClock,
+  Gift, PenTool, Umbrella,
 } from 'lucide-react'
 import Logo from '../ui/Logo'
 import { settingsApi } from '../../lib/api-services'
@@ -55,6 +56,14 @@ const navGroups: NavGroup[] = [
       { path: '/salary-dashboard', label: '薪酬分析', icon: BarChart3 },
     ],
   },
+  {
+    title: '福利保障',
+    items: [
+      { path: '/commercial-insurance', label: '商业保险', icon: Umbrella },
+      { path: '/benefits', label: '员工福利', icon: Gift },
+      { path: '/esign', label: '电子签署', icon: PenTool },
+    ],
+  },
   {
     title: '时间',
     items: [