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/habit_entry.go

@@ -0,0 +1,30 @@
+package entities
+
+import "time"
+
+type HabitEntry struct {
+	ID            string
+	HabitID       string
+	ScheduledDate time.Time
+	CompletedAt   time.Time
+	Value         *float64
+	DeletedAt     *time.Time
+}
+
+func NewHabitEntry(habitID string, scheduledDate time.Time, value *float64) *HabitEntry {
+	return &HabitEntry{
+		HabitID:       habitID,
+		ScheduledDate: scheduledDate,
+		CompletedAt:   time.Now(),
+		Value:         value,
+	}
+}
+
+func (e *HabitEntry) Delete() {
+	now := time.Now()
+	e.DeletedAt = &now
+}
+
+func (e *HabitEntry) IsDeleted() bool {
+	return e.DeletedAt != nil
+}