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/user.go

@@ -0,0 +1,23 @@
+package entities
+
+import "time"
+
+type User struct {
+	ID           string
+	Email        string
+	PasswordHash string
+	Timezone     string
+	CreatedAt    time.Time
+	UpdatedAt    time.Time
+}
+
+func NewUser(email, passwordHash, timezone string) *User {
+	now := time.Now()
+	return &User{
+		Email:        email,
+		PasswordHash: passwordHash,
+		Timezone:     timezone,
+		CreatedAt:    now,
+		UpdatedAt:    now,
+	}
+}