Update TestAccountDefaultValues assertion to match zh_CN default locale

Account.Locale gorm default was changed from 'en' to 'zh_CN' in the
locale cleanup commit (a1ae852), but the default-values test still
asserted 'en', causing TestAccountDefaultValues to fail. Align the
assertion with the new default.

backend/internal/model/constraints_test.go

@@ -19,7 +19,7 @@ func TestAccountDefaultValues(t *testing.T) {
 
 	var fetched model.Account
 	assert.NoError(t, db.First(&fetched, acc.ID).Error)
-	assert.Equal(t, "en", fetched.Locale, "Locale should default to 'en'")
+	assert.Equal(t, "zh_CN", fetched.Locale, "Locale should default to 'zh_CN'")
 	assert.Equal(t, "UTC", fetched.Timezone, "Timezone should default to 'UTC'")
 	assert.True(t, fetched.Active, "Active should default to true")
 	assert.Equal(t, "active", fetched.Status, "Status should default to 'active'")