fix: make -o flag robust against PowerShell argument splitting

PowerShell splits `-o=file.xlsx` into `-o=file` + `.xlsx` as separate
args. Fix: auto-append .xlsx if output has no extension, and filter
out .xlsx files from input args with a helpful warning.

main.go

@@ -28,9 +28,15 @@ func main() {
 		os.Exit(0)
 	}
 
+	// PowerShell kann "-o=testdatei.xlsx" in "-o=testdatei" + ".xlsx" splitten
+	if filepath.Ext(*out) == "" {
+		*out += ".xlsx"
+	}
+
 	files := flag.Args()
 	if len(files) == 0 {
-		fmt.Fprintln(os.Stderr, "Verwendung: csv2xlsx [flags] datei1.csv datei2.csv ...")
+		fmt.Fprintln(os.Stderr, "Verwendung: csv2excel [flags] datei1.csv datei2.csv ...")
+		fmt.Fprintln(os.Stderr, "Tipp: In PowerShell Leerzeichen statt = verwenden: -o ausgabe.xlsx")
 		os.Exit(1)
 	}
 
@@ -44,6 +50,19 @@ func run(files []string, out, sep, enc string) error {
 	xlsx := excelize.NewFile()
 	firstSheet := true
 
+	var csvFiles []string
+	for _, f := range files {
+		if strings.EqualFold(filepath.Ext(f), ".xlsx") {
+			fmt.Fprintf(os.Stderr, "Warnung: %s übersprungen – Excel-Datei als Input? Meintest du: -o=%s\n", f, f)
+			continue
+		}
+		csvFiles = append(csvFiles, f)
+	}
+	files = csvFiles
+	if len(files) == 0 {
+		return fmt.Errorf("keine CSV-Dateien zum Verarbeiten")
+	}
+
 	for _, path := range files {
 
 		sheetName := strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))