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/lib/mock.ts

@@ -22,10 +22,7 @@ export const menuSections = [
 
 export const menuItems = [
   { id: 'dashboard', label: '仪表盘', icon: 'LayoutDashboard', section: 'overview', roles: ['admin', 'dba', 'developer', 'guest'] },
-  { id: 'mysql', label: 'MySQL', icon: 'Database', section: 'datasource', roles: ['admin', 'dba'] },
-  { id: 'postgres', label: 'PostgreSQL', icon: 'Server', section: 'datasource', roles: ['admin', 'dba'] },
-  { id: 'redis', label: 'Redis', icon: 'Zap', section: 'datasource', roles: ['admin', 'dba'] },
-  { id: 'mongo', label: 'MongoDB', icon: 'Leaf', section: 'datasource', roles: ['admin', 'dba'] },
+  { id: 'instances', label: '数据源', icon: 'Database', section: 'datasource', roles: ['admin', 'dba'] },
   { id: 'tickets', label: '数据变更', icon: 'ClipboardList', section: 'ticket', roles: ['admin', 'dba', 'developer'] },
   { id: 'queries', label: '查询审计', icon: 'Search', section: 'ticket', roles: ['admin', 'dba', 'developer', 'guest'] },
   { id: 'roles', label: '角色权限', icon: 'UserCog', section: 'permission', roles: ['admin'] },