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/value_objects/frequency.go

@@ -0,0 +1,17 @@
+package value_objects
+
+type Frequency string
+
+const (
+	FrequencyDaily   Frequency = "DAILY"
+	FrequencyWeekly  Frequency = "WEEKLY"
+	FrequencyMonthly Frequency = "MONTHLY"
+)
+
+func (f Frequency) IsValid() bool {
+	switch f {
+	case FrequencyDaily, FrequencyWeekly, FrequencyMonthly:
+		return true
+	}
+	return false
+}

internal/domain/value_objects/habit_type.go

@@ -0,0 +1,17 @@
+package value_objects
+
+type HabitType string
+
+const (
+	HabitTypeBoolean HabitType = "BOOLEAN"
+	HabitTypeCounter HabitType = "COUNTER"
+	HabitTypeValue   HabitType = "VALUE"
+)
+
+func (ht HabitType) IsValid() bool {
+	switch ht {
+	case HabitTypeBoolean, HabitTypeCounter, HabitTypeValue:
+		return true
+	}
+	return false
+}