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/totals.go

@@ -17,7 +17,6 @@ const (
 type DailyTotals struct {
 	TotalSeconds int64
 	BreakSeconds int64
-	PairCount    int
 }
 
 func ComputeDailyTotals(events []db.Event, minBreakThreshold int64, dayStart, dayEnd int64) DailyTotals {
@@ -41,7 +40,6 @@ func ComputeDailyTotals(events []db.Event, minBreakThreshold int64, dayStart, da
 
 	var workTotal, breakTotal int64
 	var workStart, breakStart int64
-	pairs := 0
 	state := StateIdle
 
 	for _, e := range sorted {
@@ -66,7 +64,6 @@ func ComputeDailyTotals(events []db.Event, minBreakThreshold int64, dayStart, da
 			case StateWorking:
 				workTotal += e.OccurredAt - workStart
 				workStart = 0
-				pairs++
 				breakStart = e.OccurredAt
 				state = StateOnBreak
 			case StateIdle:
@@ -81,6 +78,5 @@ func ComputeDailyTotals(events []db.Event, minBreakThreshold int64, dayStart, da
 	return DailyTotals{
 		TotalSeconds: workTotal,
 		BreakSeconds: breakTotal,
-		PairCount:    pairs,
 	}
 }