Implement domain layer (TDD - Green phase)

Implementation to make all tests pass:

Value Objects:
- HabitType: BOOLEAN, COUNTER, VALUE with validation
- Frequency: DAILY, WEEKLY, MONTHLY with validation

Entities:
- User: email, password hash, timezone with defaults
- Habit: tracking habits with type, frequency, carry-over
- HabitEntry: recording habit completions with soft delete

Repositories (interfaces):
- UserRepository: CRUD operations for users
- HabitRepository: CRUD operations for habits
- HabitEntryRepository: CRUD operations for entries

Shared:
- Common error definitions for domain layer

All domain tests now pass.

internal/domain/entities/habit.go

@@ -0,0 +1,48 @@
+package entities
+
+import (
+	"time"
+
+	"habit-tracker-api/internal/domain/value_objects"
+)
+
+type Habit struct {
+	ID            string
+	UserID        string
+	Name          string
+	Description   string
+	Type          value_objects.HabitType
+	Frequency     value_objects.Frequency
+	SpecificDays  []int
+	SpecificDates []int
+	CarryOver     bool
+	TargetValue   *float64
+	CreatedAt     time.Time
+	ArchivedAt    *time.Time
+}
+
+func NewHabit(
+	userID string,
+	name string,
+	habitType value_objects.HabitType,
+	frequency value_objects.Frequency,
+	carryOver bool,
+) *Habit {
+	return &Habit{
+		UserID:    userID,
+		Name:      name,
+		Type:      habitType,
+		Frequency: frequency,
+		CarryOver: carryOver,
+		CreatedAt: time.Now(),
+	}
+}
+
+func (h *Habit) Archive() {
+	now := time.Now()
+	h.ArchivedAt = &now
+}
+
+func (h *Habit) IsActive() bool {
+	return h.ArchivedAt == nil
+}