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/mark_habit.go

@@ -0,0 +1,50 @@
+package commands
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"habit-tracker-api/internal/domain/entities"
+	"habit-tracker-api/internal/domain/repositories"
+)
+
+type MarkHabitCommand struct {
+	HabitID       string
+	ScheduledDate time.Time
+	Value         *float64
+}
+
+type MarkHabitHandler struct {
+	entryRepo repositories.HabitEntryRepository
+	habitRepo repositories.HabitRepository
+}
+
+func NewMarkHabitHandler(
+	entryRepo repositories.HabitEntryRepository,
+	habitRepo repositories.HabitRepository,
+) *MarkHabitHandler {
+	return &MarkHabitHandler{
+		entryRepo: entryRepo,
+		habitRepo: habitRepo,
+	}
+}
+
+func (h *MarkHabitHandler) Handle(ctx context.Context, cmd MarkHabitCommand) error {
+	habit, err := h.habitRepo.FindByID(ctx, cmd.HabitID)
+	if err != nil {
+		return err
+	}
+
+	if habit == nil {
+		return fmt.Errorf("habit not found")
+	}
+
+	if !habit.IsActive() {
+		return fmt.Errorf("habit is archived")
+	}
+
+	entry := entities.NewHabitEntry(cmd.HabitID, cmd.ScheduledDate, cmd.Value)
+
+	return h.entryRepo.Create(ctx, entry)
+}