fix: agent_version 为空 & 恢复批量卸载

- 后端上报时若 agent_version 为空则不覆盖已有版本（防止旧Agent清空版本号）
- 安装/升级后写入版本号 v0.1.0（而非 installed/latest）
- DB 存量 online 主机 agent_version 补填 v0.1.0
- 前端恢复批量卸载 Agent 按钮

backend/internal/api/handlers/agent.go

@@ -96,19 +96,30 @@ func (h *AgentHandler) ReceiveReport(c *gin.Context) {
 	}
 
 	// 直接 SQL 更新 asset_servers 监控字段
+	// 若 AgentVersion 非空则同步写入，否则保留数据库中已有版本号（防止旧版Agent空字段覆盖已设置的版本）
 	updateSQL := `UPDATE asset_servers SET 
 		cpu_usage = ?, memory_usage = ?, disk_usage = ?,
 		cpu = ?, memory = ?, disk = ?,
-		status = ?, agent_status = ?, agent_version = ?,
+		status = ?, agent_status = ?` + func() string {
+		if req.AgentVersion != "" {
+			return `, agent_version = ?`
+		}
+		return ``
+	}() + `,
 		hostname = CASE WHEN hostname != ? AND ip = ? THEN ? ELSE hostname END,
 		updated_at = NOW()`
 	args := []interface{}{
 		req.CPU.UsagePercent, req.Memory.UsagePercent, diskAvg,
 		req.CPU.CoreCount, memMB, diskGB,
-		"running", "online", req.AgentVersion,
+		"running", "online",
+	}
+	if req.AgentVersion != "" {
+		args = append(args, req.AgentVersion)
+	}
+	args = append(args,
 		// CASE WHEN hostname != ? AND ip = ? THEN ? ELSE hostname END
 		req.Hostname, matchIP, req.Hostname,
-	}
+	)
 	if req.System != nil {
 		if req.System.OSVersion != "" {
 			updateSQL += ", os = ?"