improvement: error handeling structure changed and unified all across the app. user no longer sees inappropriate messages.

backend/src/modules/staff/dto/accept-staff-invite.dto.ts

@@ -1,14 +1,16 @@
 import { IsString, MinLength } from 'class-validator';
+import { ErrorCode } from '../../../common/errors';
 
 export class AcceptStaffInviteDto {
   @IsString()
+  @MinLength(1, { message: ErrorCode.VALIDATION_TOKEN_REQUIRED })
   token: string;
 
   @IsString()
-  @MinLength(8)
+  @MinLength(8, { message: ErrorCode.VALIDATION_PASSWORD_TOO_SHORT })
   password: string;
 
   @IsString()
-  @MinLength(1)
+  @MinLength(1, { message: ErrorCode.VALIDATION_NAME_TOO_SHORT })
   name: string;
 }