refactor: 社保公积金从薪税tab移至顶层导航，放在薪税和解聘之间

- 新建 SocialInsurance.tsx 独立页面
- App.tsx 添加 /social 路由
- TopNav/MobileTabBar 添加社保公积金导航项
- Money.tsx 移除 social tab 和 SocialInsuranceCalculator 函数

frontend/src/components/layout/MobileTabBar.tsx

@@ -1,11 +1,12 @@
 import { Link, useLocation } from 'react-router-dom'
-import { Home, FileText, Users, Calculator, UserX, Bot } from 'lucide-react'
+import { Home, FileText, Users, Calculator, UserX, Bot, Shield } from 'lucide-react'
 import clsx from 'clsx'
 
 const tabs = [
   { path: '/', label: '总览', icon: Home },
   { path: '/roster', label: '花名册', icon: Users },
   { path: '/money', label: '薪税', icon: Calculator },
+  { path: '/social', label: '社保', icon: Shield },
   { path: '/termination', label: '解聘', icon: UserX },
   { path: '/ai-assistant', label: 'AI', icon: Bot },
 ]

frontend/src/components/layout/TopNav.tsx

@@ -8,6 +8,7 @@ const tabs = [
   { path: '/', label: '总览' },
   { path: '/roster', label: '花名册' },
   { path: '/money', label: '薪税' },
+  { path: '/social', label: '社保公积金' },
   { path: '/termination', label: '解聘' },
   { path: '/ai-assistant', label: 'AI顾问' },
 ]