test(config): erp-config 从 50 增至 66 个单元测试 — fallback_chain + model_to_resp + ThemeResp

- setting_service: 7 个测试（5 个 fallback_chain 作用域解析 + 2 个 model_to_resp 映射）
- theme_handler: 2 个测试（default_theme 默认值 + ThemeResp serde round-trip）

crates/erp-config/src/handler/theme_handler.rs

@@ -106,3 +106,30 @@ where
 
     Ok(JsonResponse(ApiResponse::ok(req)))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_theme_all_fields_none() {
+        let theme = default_theme();
+        assert!(theme.primary_color.is_none());
+        assert!(theme.logo_url.is_none());
+        assert!(theme.sidebar_style.is_none());
+    }
+
+    #[test]
+    fn theme_resp_serde_roundtrip() {
+        let theme = ThemeResp {
+            primary_color: Some("#1890ff".to_string()),
+            logo_url: None,
+            sidebar_style: Some("dark".to_string()),
+        };
+        let json = serde_json::to_string(&theme).unwrap();
+        let back: ThemeResp = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.primary_color, Some("#1890ff".to_string()));
+        assert_eq!(back.logo_url, None);
+        assert_eq!(back.sidebar_style, Some("dark".to_string()));
+    }
+}