style(core): 调整表格头默认样式

- 修改表格头默认字体颜色为黑色 (#000000)
- 修改表格头默认背景颜色为灰色 (#cccccc)
- 更新相关服务类、工具类和注解的默认值

src/main/java/cn/isliu/core/annotation/TableConf.java

@@ -49,12 +49,12 @@ public @interface TableConf {
      *
      * @return 字体颜色
      */
-    String headFontColor() default "#ffffff";
+    String headFontColor() default "#000000";
 
     /**
      * 背景颜色
      *
      * @return 背景颜色
      */
-    String headBackColor() default "#000000";
+    String headBackColor() default "#cccccc";
 }

src/main/java/cn/isliu/core/service/CustomCellService.java

@@ -819,8 +819,8 @@ public class CustomCellService extends AbstractFeishuApiService {
         private String formatter= "";
         private Integer hAlign = 1;
         private Integer vAlign = 1;
-        private String foreColor = "#ffffff";
+        private String foreColor = "#000000";
-        private String backColor = "#000000";
+        private String backColor = "#cccccc";
         private String borderType = "FULL_BORDER";
         private String borderColor = "#6d6d6d";
         private Boolean clean = false;

src/main/java/cn/isliu/core/utils/PropertyUtil.java

@@ -408,12 +408,12 @@ public class PropertyUtil {
 
                 @Override
                 public String headFontColor() {
-                    return "#ffffff";
+                    return "#000000";
                 }
 
                 @Override
                 public String headBackColor() {
-                    return "#000000";
+                    return "#cccccc";
                 }
             };
         }