docs: update README, API reference, dev-log, project structure with Agent monitoring + quick-alarm + latest fixes

- README.md: React 19, default pwd 123456, Docker Compose deploy, correct module list
- api-reference.md: add Agent monitoring section (report/latest/rules/quick-setup/alerts/tasks/kline)
- dev-log.md: new entry documenting hostname/IP fix, system collector, offline detection, quick alarm
- project-structure.md: add agent/ directory, dev-log.md link, React 19

frontend/src/components/layout/Sidebar.tsx

@@ -88,9 +88,8 @@ const menuItems: MenuItem[] = [
     children: [
       { id: 'grafana', name: '监控大屏', icon: Monitor, path: '/dashboards' },
       { id: 'monitor-center', name: '资源监控', icon: Activity, path: '/monitor-center' },
-      { id: 'metrics', name: '指标监控', icon: Cpu, path: '/metrics' },
       { id: 'trace', name: '链路追踪', icon: GitBranch, path: '/trace' },
-      { id: 'alert-center', name: '告警服务', icon: Bell, path: '/alert-center' },
+      { id: 'alert-center', name: '告警中心', icon: Bell, path: '/alert-center' },
       { id: 'datasources', name: '数据源', icon: Database, path: '/datasources' },
     ],
   },