Refactor logging to use slog package across the application

- Replaced standard log package with log/slog in httpclient, nginx manager, sync service, updater, and other components for structured logging.
- Introduced environment variable `LOG_LEVEL` to control logging levels (debug, info, warn, error).
- Updated documentation to reflect changes in logging configuration and requirements.
- Added a new logging setup function in the ats_agent internal package to initialize the slog logger.

atsf_server/common/utils.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"github.com/google/uuid"
 	"html/template"
-	"log"
 	"net"
 	"os/exec"
 	"runtime"
@@ -24,14 +23,14 @@ func OpenBrowser(url string) {
 		err = exec.Command("open", url).Start()
 	}
 	if err != nil {
-		log.Println(err)
+		SysError(err.Error())
 	}
 }
 
 func GetIp() (ip string) {
 	ips, err := net.InterfaceAddrs()
 	if err != nil {
-		log.Println(err)
+		SysError(err.Error())
 		return ip
 	}