feat(admin): add platform-specific admin privilege checks

- Upgrade Go version from 1.21 to 1.25.0
- Update golang.org/x/sys dependency to v0.42.0
- Add Unix/Linux admin check using os.Geteuid() with sudo requirement
- Add Windows admin check using windows.SID and token membership validation
- Integrate admin privilege validation into main CLI entry point
- Enhance monitor.go with graceful signal handling for Ctrl+C interrupts
- Add signal channels for clean shutdown of monitoring loop
- Ensures VPN client runs with required elevated privileges on both platforms

main_cli.go

@@ -4,11 +4,15 @@ import (
 	"fmt"
 	"os"
 
+	"vpn-client/internal/admin"
 	"vpn-client/internal/cli"
 	"vpn-client/internal/config"
 )
 
 func main() {
+	// Проверка прав администратора
+	admin.RequireAdmin()
+
 	// Инициализация конфигурации
 	if err := config.Init(); err != nil {
 		fmt.Fprintf(os.Stderr, "Ошибка инициализации: %v\n", err)