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/queries/get_todays_habits.go

@@ -0,0 +1,92 @@
+package queries
+
+import (
+	"context"
+	"time"
+
+	"habit-tracker-api/internal/domain/repositories"
+	"habit-tracker-api/internal/shared/utils"
+)
+
+type TodaysHabitDTO struct {
+	ID            string
+	Name          string
+	Type          string
+	TargetValue   *float64
+	ScheduledDate time.Time
+	IsCarriedOver bool
+}
+
+type GetTodaysHabitsQuery struct {
+	UserID   string
+	Timezone string
+	Date     time.Time
+}
+
+type GetTodaysHabitsHandler struct {
+	habitRepo repositories.HabitRepository
+	entryRepo repositories.HabitEntryRepository
+}
+
+func NewGetTodaysHabitsHandler(
+	habitRepo repositories.HabitRepository,
+	entryRepo repositories.HabitEntryRepository,
+) *GetTodaysHabitsHandler {
+	return &GetTodaysHabitsHandler{
+		habitRepo: habitRepo,
+		entryRepo: entryRepo,
+	}
+}
+
+func (h *GetTodaysHabitsHandler) Handle(
+	ctx context.Context,
+	query GetTodaysHabitsQuery,
+) ([]TodaysHabitDTO, error) {
+	habits, err := h.habitRepo.FindActiveByUserID(ctx, query.UserID)
+	if err != nil {
+		return nil, err
+	}
+
+	var result []TodaysHabitDTO
+
+	for _, habit := range habits {
+		shouldAppear := utils.ShouldAppearToday(
+			string(habit.Frequency),
+			habit.SpecificDays,
+			habit.SpecificDates,
+			query.Date,
+		)
+
+		if !shouldAppear && !habit.CarryOver {
+			continue
+		}
+
+		entries, _ := h.entryRepo.FindByHabitIDAndDateRange(
+			ctx,
+			habit.ID,
+			query.Date.AddDate(0, 0, -30),
+			query.Date,
+		)
+
+		isCompleted := false
+		for _, entry := range entries {
+			if entry.ScheduledDate.Equal(query.Date) && entry.DeletedAt == nil {
+				isCompleted = true
+				break
+			}
+		}
+
+		if !isCompleted {
+			result = append(result, TodaysHabitDTO{
+				ID:            habit.ID,
+				Name:          habit.Name,
+				Type:          string(habit.Type),
+				TargetValue:   habit.TargetValue,
+				ScheduledDate: query.Date,
+				IsCarriedOver: !shouldAppear && habit.CarryOver,
+			})
+		}
+	}
+
+	return result, nil
+}