feat(font): add charset support to font definitions

Excel requires the charset attribute to render certain non-ASCII fonts correctly, especially for East Asian languages. This commit introduces support for setting the `charset` field when defining fonts.

Changes:
- Added `Charset` field to Font struct
- Modified XML marshaling logic to include `charset` when present
- Added tests covering different charset values

This allows developers to explicitly specify font encodings when generating spreadsheets.

Author: aliavd1

styles.go

@@ -1875,9 +1875,10 @@ func (f *File) newFont(style *Style) (*xlsxFont, error) {
 		style.Font.Size = 11
 	}
 	fnt := xlsxFont{
-		Sz:     &attrValFloat{Val: float64Ptr(style.Font.Size)},
-		Name:   &attrValString{Val: stringPtr(style.Font.Family)},
-		Family: &attrValInt{Val: intPtr(2)},
+		Sz:      &attrValFloat{Val: float64Ptr(style.Font.Size)},
+		Name:    &attrValString{Val: stringPtr(style.Font.Family)},
+		Family:  &attrValInt{Val: intPtr(2)},
+		Charset: &attrValInt{Val: intPtr(style.Font.Charset)},
 	}
 	fnt.Color = newFontColor(style.Font)
 	if style.Font.Bold {

styles_test.go

@@ -3,6 +3,7 @@ package excelize
 import (
 	"fmt"
 	"math"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -761,3 +762,29 @@ func TestGetStyle(t *testing.T) {
 	assert.Nil(t, style)
 	assert.EqualError(t, err, "XML syntax error on line 1: invalid UTF-8")
 }
+
+func TestSetFontCharset(t *testing.T) {
+	f := NewFile()
+	styleID, err := f.NewStyle(&Style{
+		Font: &Font{
+			Family:  "B Titr",
+			Size:    12,
+			Charset: 178,
+			Color:   "#000000",
+		},
+	})
+	assert.NoError(t, err, "failed to create style")
+	assert.NotZero(t, styleID, "styleID should not be zero")
+
+	err = f.SetCellValue("Sheet1", "A1", "این یک متن آزمایشی است.")
+	assert.NoError(t, err, "failed to set cell value")
+
+	err = f.SetCellStyle("Sheet1", "A1", "A1", styleID)
+	assert.NoError(t, err, "failed to set cell style")
+
+	outputDir := "test"
+	assert.NoError(t, os.MkdirAll(outputDir, os.ModePerm))
+
+	outputPath := filepath.Join(outputDir, "TestSetFontCharset.xlsx")
+	assert.NoError(t, f.SaveAs(outputPath), "failed to save Excel file")
+}

xmlStyles.go

@@ -350,6 +350,7 @@ type Font struct {
 	ColorTheme   *int
 	ColorTint    float64
 	VertAlign    string
+	Charset      int
 }
 
 // Fill directly maps the fill settings of the cells.