bugfix: appointment hours now use the client timezone on UTC servers.

Logical API errors throw stable codes so users see translated messages instead of a generic bad request.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/modules/appointments/dto/column-providers-query.dto.ts

@@ -1,9 +1,10 @@
 import { IsOptional, IsString, Matches } from 'class-validator';
+import { ErrorCode } from '../../../common/errors';
 
 export class ColumnProvidersQueryDto {
   /** Local calendar date (YYYY-MM-DD) used to resolve weekday working hours. */
   @IsOptional()
   @IsString()
-  @Matches(/^\d{4}-\d{2}-\d{2}$/)
+  @Matches(/^\d{4}-\d{2}-\d{2}$/, { message: ErrorCode.APPOINTMENT_INVALID_DATE })
   date?: string;
 }