refactor: 数据源菜单合一+布局优化

- 4个数据源菜单(MySQL/PG/Redis/Mongo)合并为1个「数据源」
- InstancePage 改为统一入口，顶部Tab切换类型过滤
- 添加数据源改为弹框Modal（默认MySQL）
- 去掉左下角用户区，退出登录移到右上角header
- 侧边栏收窄220px，整体布局更紧凑
- 后端202 tests passing
- 前端构建零错误

frontend/src/App.tsx

@@ -15,11 +15,12 @@ import WorkflowPage from '@/pages/WorkflowPage';
 import AutoTaskPage from '@/pages/AutoTaskPage';
 import SettingsPage from '@/pages/SettingsPage';
 import AuditLogsPage from '@/pages/AuditLogsPage';
+import DataDictionaryPage from '@/pages/DataDictionaryPage';
 
 // 路由守卫：未登录跳转登录页，无权限跳转仪表盘
 function RouteGuard() {
   const { isLoggedIn, hasPermission } = useAuth();
-  const path = location.pathname.slice(1) || 'dashboard';
+  const path = location.pathname.split('/')[1] || 'dashboard';
 
   if (!isLoggedIn) {
     return <Navigate to="/login" replace />;
@@ -50,10 +51,8 @@ function AppRoutes() {
         <Route path="/" element={<Layout />}>
           <Route index element={<Navigate to="/dashboard" replace />} />
           <Route path="dashboard" element={<Dashboard />} />
-          <Route path="mysql" element={<InstancePage />} />
-          <Route path="postgres" element={<InstancePage />} />
-          <Route path="redis" element={<InstancePage />} />
-          <Route path="mongo" element={<InstancePage />} />
+          <Route path="instances" element={<InstancePage />} />
+          <Route path="data-dictionary" element={<DataDictionaryPage />} />
           <Route path="tickets" element={<TicketsPage />} />
           <Route path="queries" element={<QueriesPage />} />
           <Route path="roles" element={<RolesPage />} />