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/repositories/user_repository.go

@@ -0,0 +1,14 @@
+package repositories
+
+import (
+	"context"
+
+	"habit-tracker-api/internal/domain/entities"
+)
+
+type UserRepository interface {
+	Create(ctx context.Context, user *entities.User) error
+	FindByID(ctx context.Context, id string) (*entities.User, error)
+	FindByEmail(ctx context.Context, email string) (*entities.User, error)
+	Update(ctx context.Context, user *entities.User) error
+}