fix(character): 防止用户重复添加相同键盘人设

在添加人设前检查是否已存在，若存在则抛REPEATEDLY_ADDING_CHARACTER异常；
同时移除实体中冗余sort字段，并统一发送验证码DTO为SendMailDTO。

src/main/java/com/yolo/keyborad/controller/UserController.java

@@ -6,6 +6,7 @@ import com.yolo.keyborad.common.BaseResponse;
 import com.yolo.keyborad.common.ResultUtils;
 import com.yolo.keyborad.model.dto.AppleLoginReq;
 import com.yolo.keyborad.model.dto.user.KeyboardUserReq;
+import com.yolo.keyborad.model.dto.user.SendMailDTO;
 import com.yolo.keyborad.model.dto.user.UserLoginDTO;
 import com.yolo.keyborad.model.dto.user.UserRegisterDTO;
 import com.yolo.keyborad.model.entity.KeyboardUser;
@@ -89,8 +90,8 @@ public class UserController {
 
     @PostMapping("/sendVerifyMail")
     @Operation(summary = "发送验证码",description = "发送验证码接口")
-    public BaseResponse<Boolean> sendVerifyMail(@RequestBody UserRegisterDTO userRegisterDTO) {
-        userService.sendVerifyMail(userRegisterDTO);
+    public BaseResponse<Boolean> sendVerifyMail(@RequestBody SendMailDTO mailDTO) {
+        userService.sendVerifyMail(mailDTO);
         return ResultUtils.success(true);
     }