Improve type safety with proper value objects for HabitType and Frequency

Replace generic string types with strongly-typed value objects throughout
the application layer. This change ensures compile-time type checking and
automatic validation during JSON deserialization.

Changes:
- Add JSON marshaling/unmarshaling to HabitType and Frequency value objects
- Update all DTOs to use typed fields instead of strings
- Update commands and queries to use proper types
- Remove unnecessary string conversions
- Add comprehensive JSON serialization tests
- Fix existing tests to work with typed fields

Benefits:
- Type safety: compiler catches invalid usage
- Automatic validation: invalid values rejected during JSON parsing
- Better code documentation and self-explanatory APIs
- Reduced runtime errors

internal/application/queries/get_habit_by_id.go

@@ -35,8 +35,8 @@ func (h *GetHabitByIDHandler) Handle(ctx context.Context, query GetHabitByIDQuer
 	return &HabitDTO{
 		ID:           habit.ID,
 		Name:         habit.Name,
-		Type:         string(habit.Type),
-		Frequency:    string(habit.Frequency),
+		Type:         habit.Type,
+		Frequency:    habit.Frequency,
 		TargetValue:  habit.TargetValue,
 		CarryOver:    habit.CarryOver,
 		SpecificDays: habit.SpecificDays,