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/hooks/useAuth.tsx

@@ -36,10 +36,8 @@ const roleNameMap: Record<UserRole, string> = {
 
 const routePermissionMap: Record<string, UserRole[]> = {
   dashboard: ['admin', 'dba', 'developer', 'guest'],
-  mysql: ['admin', 'dba'],
-  postgres: ['admin', 'dba'],
-  redis: ['admin', 'dba'],
-  mongo: ['admin', 'dba'],
+  instances: ['admin', 'dba'],
+  'data-dictionary': ['admin', 'dba', 'developer'],
   tickets: ['admin', 'dba', 'developer'],
   queries: ['admin', 'dba', 'developer', 'guest'],
   roles: ['admin'],