feat: 监控大屏 /grafana→/dashboards + Agent主机Top10默认面板

- 路由: /grafana → /dashboards (Sidebar + App.tsx)
- GrafanaDashboard 新增默认面板: Agent 主机监控 Top 10
  - CPU/Memory/Disk 使用率三列排行
  - 进度条颜色: >85%红 >60%黄 <60%绿
  - 🥇🥈🥉 emoji 前三名
  - 30s 自动刷新
- Prometheus/Zabbix/SkyWalking 需配置数据源后才可用

frontend/src/components/layout/Sidebar.tsx

@@ -88,7 +88,7 @@ const menuItems: MenuItem[] = [
     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: 'grafana', name: '监控大屏', icon: Monitor, path: '/grafana' },
+      { id: 'grafana', name: '监控大屏', icon: Monitor, path: '/dashboards' },
       { id: 'trace', name: '链路追踪', icon: GitBranch, path: '/trace' },
       { id: 'alert-center', name: '告警服务', icon: Bell, path: '/alert-center' },
     ],