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/habit_entry_test.go

@@ -0,0 +1,67 @@
+package entities
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNewHabitEntry(t *testing.T) {
+	habitID := "habit-123"
+	scheduledDate := time.Now().Truncate(24 * time.Hour)
+	value := 5.0
+
+	entry := NewHabitEntry(habitID, scheduledDate, &value)
+
+	if entry.HabitID != habitID {
+		t.Errorf("Expected HabitID %s, got %s", habitID, entry.HabitID)
+	}
+
+	if !entry.ScheduledDate.Equal(scheduledDate) {
+		t.Errorf("Expected ScheduledDate %v, got %v", scheduledDate, entry.ScheduledDate)
+	}
+
+	if entry.Value == nil {
+		t.Error("Value should not be nil")
+	}
+
+	if *entry.Value != value {
+		t.Errorf("Expected Value %f, got %f", value, *entry.Value)
+	}
+
+	if entry.CompletedAt.IsZero() {
+		t.Error("CompletedAt should not be zero")
+	}
+
+	if entry.DeletedAt != nil {
+		t.Error("DeletedAt should be nil for new entry")
+	}
+}
+
+func TestNewHabitEntry_BooleanHabit(t *testing.T) {
+	habitID := "habit-123"
+	scheduledDate := time.Now().Truncate(24 * time.Hour)
+
+	entry := NewHabitEntry(habitID, scheduledDate, nil)
+
+	if entry.Value != nil {
+		t.Error("Value should be nil for boolean habit")
+	}
+}
+
+func TestHabitEntry_SoftDelete(t *testing.T) {
+	entry := NewHabitEntry("habit-123", time.Now(), nil)
+
+	if entry.DeletedAt != nil {
+		t.Error("New entry should not be deleted")
+	}
+
+	entry.SoftDelete()
+
+	if entry.DeletedAt == nil {
+		t.Error("Entry should be deleted after calling SoftDelete()")
+	}
+
+	if entry.DeletedAt.After(time.Now()) {
+		t.Error("DeletedAt should not be in the future")
+	}
+}