fix(entity): 修正CmsNavigation中banner字段的注解描述

- 将banner字段的描述由“banner”修改为“栏目背景图/banner”
- 移除banner字段上的@TableField(exist = false)注解

src/main/java/com/gxwebsoft/cms/entity/CmsNavigation.java

@@ -69,8 +69,7 @@ public class CmsNavigation implements Serializable {
     @Schema(description = "菜单图标")
     private String icon;
 
-    @Schema(description = "banner")
-    @TableField(exist = false)
+    @Schema(description = "栏目背景图/banner")
     private String banner;
 
     @Schema(description = "移动端banner")