[management,signal,proxy,relay,misc] Unify service configuration loading

Service entry points currently resolve defaults, files, environment variables, and flags differently, which makes precedence inconsistent and prevents some services from using config files.

Introduce one Viper-backed loader and migrate Combined, Management, Relay, Signal, and Proxy while preserving compatibility aliases and Management template expansion.

signal/cmd/root.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
-	"runtime"
 
 	"github.com/spf13/cobra"
 
@@ -17,9 +16,8 @@ const (
 )
 
 var (
-	logLevel       string
-	defaultLogFile string
-	logFile        string
+	logLevel string
+	logFile  string
 
 	rootCmd = &cobra.Command{
 		Use:     "netbird-signal",
@@ -39,14 +37,9 @@ func Execute() error {
 
 func init() {
 	stopCh = make(chan int)
-	defaultLogFile = "/var/log/netbird/signal.log"
-
-	if runtime.GOOS == "windows" {
-		defaultLogFile = os.Getenv("PROGRAMDATA") + "\\Netbird\\" + "signal.log"
-	}
-
-	rootCmd.PersistentFlags().StringVar(&logLevel, "log-level", "info", "")
-	rootCmd.PersistentFlags().StringVar(&logFile, "log-file", defaultLogFile, "sets Netbird log path. If console is specified the log will be output to stdout")
+	defaults := defaultConfig()
+	rootCmd.PersistentFlags().StringVar(&logLevel, "log-level", defaults.LogLevel, "")
+	rootCmd.PersistentFlags().StringVar(&logFile, "log-file", defaults.LogFile, "sets Netbird log path. If console is specified the log will be output to stdout")
 	rootCmd.AddCommand(runCmd)
 }