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/domain/value_objects/frequency.go

@@ -1,5 +1,10 @@
 package value_objects
 
+import (
+	"encoding/json"
+	"fmt"
+)
+
 type Frequency string
 
 const (
@@ -15,3 +20,21 @@ func (f Frequency) IsValid() bool {
 	}
 	return false
 }
+
+func (f Frequency) MarshalJSON() ([]byte, error) {
+	return json.Marshal(string(f))
+}
+
+func (f *Frequency) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	*f = Frequency(s)
+	if !f.IsValid() {
+		return fmt.Errorf("invalid frequency: %s (must be DAILY, WEEKLY, or MONTHLY)", s)
+	}
+
+	return nil
+}