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_user_habits_test.go

@@ -92,11 +92,11 @@ func TestGetUserHabitsHandler_IncludesAllHabitFields(t *testing.T) {
 		t.Errorf("Expected name 'Drink Water', got %s", result.Name)
 	}
 
-	if result.Type != string(value_objects.HabitTypeValue) {
+	if result.Type != value_objects.HabitTypeValue {
 		t.Errorf("Expected type %s, got %s", value_objects.HabitTypeValue, result.Type)
 	}
 
-	if result.Frequency != string(value_objects.FrequencyDaily) {
+	if result.Frequency != value_objects.FrequencyDaily {
 		t.Errorf("Expected frequency %s, got %s", value_objects.FrequencyDaily, result.Frequency)
 	}