Add domain layer tests (TDD - Red phase)

Following TDD principles, adding tests before implementation:

- Add HabitType value object tests (validation)
- Add Frequency value object tests (validation)
- Add User entity tests (creation, default timezone)
- Add Habit entity tests (creation, archive, active status)
- Add HabitEntry entity tests (creation, soft delete)

Tests cover core domain logic and business rules.
All tests will fail until implementation is added.

internal/domain/value_objects/frequency_test.go

@@ -0,0 +1,26 @@
+package value_objects
+
+import "testing"
+
+func TestFrequency_IsValid(t *testing.T) {
+	tests := []struct {
+		name     string
+		freq     Frequency
+		expected bool
+	}{
+		{"Daily frequency is valid", FrequencyDaily, true},
+		{"Weekly frequency is valid", FrequencyWeekly, true},
+		{"Monthly frequency is valid", FrequencyMonthly, true},
+		{"Empty string is invalid", Frequency(""), false},
+		{"Random string is invalid", Frequency("YEARLY"), false},
+		{"Lowercase is invalid", Frequency("daily"), false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.freq.IsValid(); got != tt.expected {
+				t.Errorf("Frequency.IsValid() = %v, want %v", got, tt.expected)
+			}
+		})
+	}
+}

internal/domain/value_objects/habit_type_test.go

@@ -0,0 +1,26 @@
+package value_objects
+
+import "testing"
+
+func TestHabitType_IsValid(t *testing.T) {
+	tests := []struct {
+		name     string
+		habType  HabitType
+		expected bool
+	}{
+		{"Boolean type is valid", HabitTypeBoolean, true},
+		{"Counter type is valid", HabitTypeCounter, true},
+		{"Value type is valid", HabitTypeValue, true},
+		{"Empty string is invalid", HabitType(""), false},
+		{"Random string is invalid", HabitType("RANDOM"), false},
+		{"Lowercase is invalid", HabitType("boolean"), false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.habType.IsValid(); got != tt.expected {
+				t.Errorf("HabitType.IsValid() = %v, want %v", got, tt.expected)
+			}
+		})
+	}
+}