feat: 重构监控中心导航，新增资源监控4大模块

- 主菜单「监控告警」→「监控中心」
- 「告警中心」→「告警服务」移至链路追踪下方
- 新增资源监控 Tab 页（主机/中间件/SNMP/站点拨测）
- Agent 新增网络采集器(network)和磁盘IO采集器(diskio)
- 后端扩展 Agent 上报模型，支持 NetworkStat/DiskIOStat
- 前端 ServerTableRow 展示网络和磁盘IO实时指标
- WebSocket MetricsUpdate 推送扩展

frontend/src/components/layout/Sidebar.tsx

@@ -59,7 +59,6 @@ const menuItems: MenuItem[] = [
       { id: 'regions', name: '区域管理', icon: MapPin, path: '/regions' },
       { id: 'platforms', name: '云平台', icon: CloudCog, path: '/platforms' },
       { id: 'cloud-billing', name: '云账单', icon: BarChart3, path: '/cloud-billing' },
-      { id: 'agent-monitor', name: 'Agent监控', icon: Activity, path: '/agent-monitor' },
     ],
   },
   {
@@ -84,14 +83,14 @@ const menuItems: MenuItem[] = [
   },
   {
     id: 'monitor',
-    name: '监控告警',
+    name: '监控中心',
     icon: Bell,
     children: [
+      { 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: 'alert-center', name: '告警中心', icon: Bell, path: '/alert-center' },
-      { id: 'trace', name: '链路追踪', icon: GitBranch, path: '/trace' },
       { id: 'grafana', name: '监控大屏', icon: Monitor, path: '/grafana' },
-      { id: 'agent-alerts', name: 'Agent告警', icon: AlertTriangle, path: '/agent-alerts' },
+      { id: 'trace', name: '链路追踪', icon: GitBranch, path: '/trace' },
+      { id: 'alert-center', name: '告警服务', icon: Bell, path: '/alert-center' },
     ],
   },
   {