Fix Windows reload issue (/bin/sh not found)

[Alireza-Gholiei]

This PR adds native Windows compatibility for Nginx-UI, fixing the following error when reloading Nginx on
Windows:

Nginx error: exec: "/bin/sh": executable file not found in %PATH%

Updated execShell in internal/nginx/exec.go
to use cmd /C instead of /bin/sh -c when running on Windows.

This makes ReloadCmd, RestartCmd, and other Nginx commands work properly on Windows environments.

Implementation Detail

func execShell(cmd string) (stdOut string, stdErr error) {
    if runtime.GOOS == "windows" {
        return execCommand("cmd", "/C", cmd)
    }
    return execCommand("/bin/sh", "-c", cmd)
}

Added import "runtime" for OS detection.

Tested Environment

OS: Windows 10 / Windows Server 2019

Nginx: Windows build (nginx.exe)

Result:

Reload and Restart work correctly

UI responds successfully

No /bin/sh errors

[0xJacky]

LGTM 🎉