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

backend/app/api/v1/router.py

@@ -3,6 +3,7 @@ from fastapi import APIRouter
 from app.api.v1.endpoints import (
     auth, user, instance, ticket, query, dashboard, audit, audit_rule,
     announcement, notification, ldap_config,
+    instance_detail, backup, redis, dictionary,
 )
 
 api_router = APIRouter(prefix="/api/v1")
@@ -18,3 +19,7 @@ api_router.include_router(audit_rule.router)
 api_router.include_router(announcement.router)
 api_router.include_router(notification.router)
 api_router.include_router(ldap_config.router)
+api_router.include_router(instance_detail.router)
+api_router.include_router(backup.router)
+api_router.include_router(redis.router)
+api_router.include_router(dictionary.router, prefix="/dictionary", tags=["dictionary"])