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/application/commands/create_habit.go

@@ -0,0 +1,62 @@
+package commands
+
+import (
+	"context"
+
+	"habit-tracker-api/internal/domain/entities"
+	"habit-tracker-api/internal/domain/repositories"
+	"habit-tracker-api/internal/domain/value_objects"
+	"habit-tracker-api/internal/shared/errors"
+)
+
+type CreateHabitCommand struct {
+	UserID        string
+	Name          string
+	Description   string
+	Type          string
+	Frequency     string
+	SpecificDays  []int
+	SpecificDates []int
+	CarryOver     bool
+	TargetValue   *float64
+}
+
+type CreateHabitHandler struct {
+	habitRepo repositories.HabitRepository
+}
+
+func NewCreateHabitHandler(habitRepo repositories.HabitRepository) *CreateHabitHandler {
+	return &CreateHabitHandler{habitRepo: habitRepo}
+}
+
+func (h *CreateHabitHandler) Handle(ctx context.Context, cmd CreateHabitCommand) (string, error) {
+	habitType := value_objects.HabitType(cmd.Type)
+	if !habitType.IsValid() {
+		return "", errors.ErrInvalidInput
+	}
+
+	frequency := value_objects.Frequency(cmd.Frequency)
+	if !frequency.IsValid() {
+		return "", errors.ErrInvalidInput
+	}
+
+	if frequency == value_objects.FrequencyWeekly && len(cmd.SpecificDays) == 0 {
+		return "", errors.ErrInvalidInput
+	}
+
+	if frequency == value_objects.FrequencyMonthly && len(cmd.SpecificDates) == 0 {
+		return "", errors.ErrInvalidInput
+	}
+
+	habit := entities.NewHabit(cmd.UserID, cmd.Name, habitType, frequency, cmd.CarryOver)
+	habit.Description = cmd.Description
+	habit.SpecificDays = cmd.SpecificDays
+	habit.SpecificDates = cmd.SpecificDates
+	habit.TargetValue = cmd.TargetValue
+
+	if err := h.habitRepo.Create(ctx, habit); err != nil {
+		return "", err
+	}
+
+	return habit.ID, nil
+}