feat: add cross-platform support (Windows/Linux/macOS), update README

cmd/main.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"os"
 	"os/signal"
+	"runtime"
 	"syscall"
 
 	"ftp-server/config"
@@ -24,8 +25,9 @@ func main() {
 	}
 
 	fmt.Println("======================================")
-	fmt.Println("       FTP Server for Windows")
+	fmt.Println("       FTP Server (Cross-Platform)")
 	fmt.Println("======================================")
+	fmt.Printf("OS/Arch:     %s/%s\n", runtime.GOOS, runtime.GOARCH)
 	fmt.Printf("FTP Port:    %d\n", cfg.FTP.Port)
 	fmt.Printf("Web Admin:   http://127.0.0.1:%d\n", cfg.Web.Port)
 	fmt.Printf("Admin User:  %s\n", cfg.Admin.Username)