Start week on Monday

internal/handlers/handlers.go

@@ -214,9 +214,15 @@ func (h *Handlers) buildCalendarData(year int, month time.Month) map[string]any
 		days[day] = content
 	}
 
-	// Calculate first day offset (0 = Sunday)
+	// Calculate first day offset (0 = Monday, 6 = Sunday)
 	firstDay := time.Date(year, month, 1, 0, 0, 0, 0, time.Local)
 	firstDayWeekday := int(firstDay.Weekday())
+	// Adjust so Monday = 0, Sunday = 6
+	if firstDayWeekday == 0 {
+		firstDayWeekday = 6
+	} else {
+		firstDayWeekday--
+	}
 
 	// Previous/next month
 	prevMonth := time.Date(year, month-1, 1, 0, 0, 0, 0, time.Local)