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

agent/main.go

@@ -103,8 +103,9 @@ func main() {
 		// 前 3 次上报携带系统信息
 		if sysReportCount < 3 && (sysInfo.OSVersion != "" || sysInfo.Kernel != "") {
 			report.System = &model.SystemStat{
-				OSVersion: sysInfo.OSVersion,
-				Kernel:    sysInfo.Kernel,
+				OSVersion:     sysInfo.OSVersion,
+				Kernel:        sysInfo.Kernel,
+				Uptime:        sysInfo.UptimeSeconds,
 			}
 			sysReportCount++
 		}