Add domain layer tests (TDD - Red phase)

Following TDD principles, adding tests before implementation:

- Add HabitType value object tests (validation)
- Add Frequency value object tests (validation)
- Add User entity tests (creation, default timezone)
- Add Habit entity tests (creation, archive, active status)
- Add HabitEntry entity tests (creation, soft delete)

Tests cover core domain logic and business rules.
All tests will fail until implementation is added.

internal/domain/entities/user_test.go

@@ -0,0 +1,48 @@
+package entities
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNewUser(t *testing.T) {
+	email := "test@example.com"
+	passwordHash := "hashed_password_123"
+	timezone := "Europe/Madrid"
+
+	user := NewUser(email, passwordHash, timezone)
+
+	if user.Email != email {
+		t.Errorf("Expected email %s, got %s", email, user.Email)
+	}
+
+	if user.PasswordHash != passwordHash {
+		t.Errorf("Expected password hash %s, got %s", passwordHash, user.PasswordHash)
+	}
+
+	if user.Timezone != timezone {
+		t.Errorf("Expected timezone %s, got %s", timezone, user.Timezone)
+	}
+
+	if user.CreatedAt.IsZero() {
+		t.Error("CreatedAt should not be zero")
+	}
+
+	if user.UpdatedAt.IsZero() {
+		t.Error("UpdatedAt should not be zero")
+	}
+
+	// CreatedAt and UpdatedAt should be very close in time
+	diff := user.UpdatedAt.Sub(user.CreatedAt)
+	if diff < 0 || diff > time.Second {
+		t.Errorf("CreatedAt and UpdatedAt should be nearly identical, diff: %v", diff)
+	}
+}
+
+func TestUser_DefaultTimezone(t *testing.T) {
+	user := NewUser("test@example.com", "hash", "")
+
+	if user.Timezone != "UTC" {
+		t.Errorf("Expected default timezone UTC, got %s", user.Timezone)
+	}
+}