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/shared/errors/errors.go

@@ -0,0 +1,11 @@
+package errors
+
+import "errors"
+
+var (
+	ErrNotFound          = errors.New("resource not found")
+	ErrAlreadyExists     = errors.New("resource already exists")
+	ErrInvalidInput      = errors.New("invalid input")
+	ErrUnauthorized      = errors.New("unauthorized")
+	ErrInvalidCredentials = errors.New("invalid credentials")
+)