feat: Agent采集uptime, 后端持久化kernel/uptime/diskIO, 前端磁盘BPS+IOPS展示

- Agent: system.go 添加 readUptime(), SystemStat 携带 Uptime
- Backend: Server 模型新增 Kernel 字段, ReceiveReport 持久化 kernel+uptime
- Backend: /agent/latest 返回 kernel/uptime/port_info/disk_io_info/disk_total
- Frontend: 磁盘监控改为 BPS+IOPS (4张卡片), 系统信息修复内核/运行时长/监听端口/磁盘容量
- 图表单位从 MB/s 改为 KB/s, 添加 formatBPS 工具函数

frontend/src/components/layout/Sidebar.tsx

@@ -123,6 +123,7 @@ const menuItems: MenuItem[] = [
       { id: 'roles', name: '角色权限', icon: Shield, path: '/roles' },
       { id: 'logs-audit', name: '操作审计', icon: FileCode, path: '/logs-audit' },
       { id: 'terminal-audit', name: '终端审计', icon: Terminal, path: '/terminal-audit' },
+      { id: 'credentials', name: '凭证配置', icon: Shield, path: '/credentials' },
       { id: 'settings', name: '系统设置', icon: Settings, path: '/settings' },
     ],
   },