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_json_test.go

@@ -0,0 +1,90 @@
+package value_objects
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestFrequency_MarshalJSON(t *testing.T) {
+	tests := []struct {
+		name      string
+		frequency Frequency
+		expected  string
+	}{
+		{"Daily", FrequencyDaily, `"DAILY"`},
+		{"Weekly", FrequencyWeekly, `"WEEKLY"`},
+		{"Monthly", FrequencyMonthly, `"MONTHLY"`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			data, err := json.Marshal(tt.frequency)
+			if err != nil {
+				t.Fatalf("Failed to marshal: %v", err)
+			}
+			if string(data) != tt.expected {
+				t.Errorf("Expected %s, got %s", tt.expected, string(data))
+			}
+		})
+	}
+}
+
+func TestFrequency_UnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		name      string
+		input     string
+		expected  Frequency
+		shouldErr bool
+	}{
+		{"Valid Daily", `"DAILY"`, FrequencyDaily, false},
+		{"Valid Weekly", `"WEEKLY"`, FrequencyWeekly, false},
+		{"Valid Monthly", `"MONTHLY"`, FrequencyMonthly, false},
+		{"Invalid frequency", `"YEARLY"`, "", true},
+		{"Empty string", `""`, "", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var f Frequency
+			err := json.Unmarshal([]byte(tt.input), &f)
+
+			if tt.shouldErr {
+				if err == nil {
+					t.Error("Expected error but got none")
+				}
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Unexpected error: %v", err)
+			}
+
+			if f != tt.expected {
+				t.Errorf("Expected %s, got %s", tt.expected, f)
+			}
+		})
+	}
+}
+
+func TestFrequency_JSONRoundTrip(t *testing.T) {
+	type testStruct struct {
+		Frequency Frequency `json:"frequency"`
+	}
+
+	original := testStruct{Frequency: FrequencyWeekly}
+
+	data, err := json.Marshal(original)
+	if err != nil {
+		t.Fatalf("Marshal failed: %v", err)
+	}
+
+	var decoded testStruct
+	err = json.Unmarshal(data, &decoded)
+	if err != nil {
+		t.Fatalf("Unmarshal failed: %v", err)
+	}
+
+	if decoded.Frequency != original.Frequency {
+		t.Errorf("Round trip failed: expected %s, got %s", original.Frequency, decoded.Frequency)
+	}
+}