Implement Application Layer with TDD

- Add date utilities for habit scheduling logic
- Implement CreateHabitHandler with full validation
- Implement GetTodaysHabitsHandler with carry-over support
- Implement MarkHabitHandler for completing habits
- All components developed following TDD methodology
- Complete test coverage for commands and queries

internal/shared/utils/date_utils.go

@@ -0,0 +1,31 @@
+package utils
+
+import "time"
+
+func ShouldAppearToday(
+	frequency string,
+	specificDays []int,
+	specificDates []int,
+	targetDate time.Time,
+) bool {
+	switch frequency {
+	case "DAILY":
+		return true
+	case "WEEKLY":
+		weekday := int(targetDate.Weekday())
+		return contains(specificDays, weekday)
+	case "MONTHLY":
+		day := targetDate.Day()
+		return contains(specificDates, day)
+	}
+	return false
+}
+
+func contains(slice []int, val int) bool {
+	for _, item := range slice {
+		if item == val {
+			return true
+		}
+	}
+	return false
+}