refactor: remove dead code, DRY calendar navigation and helpers

- Fix Jalali calendar weekday offset in buildCalendarMonth
- Remove unused gregorianDateKey function and PairCount field
- Replace inline export month title with formatMonthTitle
- Extract navMonth helper for prev/next month navigation
- Extract sendOrEdit helper for send-vs-edit message pattern
- Extract monthGregorianRange helper to simplify editCalendar 3-way switch
- Replace export displayDate conversion with formatDateForCalendar

internal/bot/export.go

@@ -76,17 +76,16 @@ func GenerateMonthlyReport(store *db.Store, userID int64, timezone, accent, cale
 		Alignment: &excelize.Alignment{Horizontal: "center", Vertical: "center"},
 	})
 
-	var monthName string
+	var jy, jm int
 	switch calendar {
 	case "jalali":
-		jy, jm, _ := gregorianToJalali(year, int(month), 1)
-		monthName = fmt.Sprintf("%s %d", jalaliMonthNames[jm-1], jy)
+		jy, jm, _ = gregorianToJalali(year, int(month), 1)
 	case "hijri":
-		hy, hm, _ := gregorianToHijri(year, int(month), 1)
-		monthName = fmt.Sprintf("%s %d", hijriMonthNames[hm-1], hy)
+		jy, jm, _ = gregorianToHijri(year, int(month), 1)
 	default:
-		monthName = time.Date(year, month, 1, 0, 0, 0, 0, time.UTC).Format("January 2006")
+		jy, jm = year, int(month)
 	}
+	monthName := formatMonthTitle(calendar, jy, jm)
 	f.SetCellValue(sheet, "A1", fmt.Sprintf("Work Time Report - %s", monthName))
 	f.MergeCell(sheet, "A1", "F1")
 	f.SetCellStyle(sheet, "A1", "F1", titleStyle)
@@ -130,16 +129,7 @@ func GenerateMonthlyReport(store *db.Store, userID int64, timezone, accent, cale
 
 	for d := firstDay; !d.After(lastDay); d = d.AddDate(0, 0, 1) {
 		dateStr := d.Format("2006-01-02")
-		displayDate := dateStr
-		if calendar == "jalali" {
-			gy, gm, gd := parseGregorianDateKey(dateStr)
-			jy, jm, jd := gregorianToJalali(gy, gm, gd)
-			displayDate = fmt.Sprintf("%04d-%02d-%02d", jy, jm, jd)
-		} else if calendar == "hijri" {
-			gy, gm, gd := parseGregorianDateKey(dateStr)
-			hy, hm, hd := gregorianToHijri(gy, gm, gd)
-			displayDate = fmt.Sprintf("%04d-%02d-%02d", hy, hm, hd)
-		}
+		displayDate := formatDateForCalendar(dateStr, calendar)
 
 		day, exists := dayMap[dateStr]
 		if !exists {