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/treatments/lab-case-send.validation.ts

@@ -1,4 +1,5 @@
-import { BadRequestException } from '@nestjs/common';
+import { HttpStatus } from '@nestjs/common';
+import { AppException, ErrorCode } from '../../common/errors';
 import { normalizeTeeth } from './treatment.utils';
 
 export type LabCaseProsthesisLink = {
@@ -28,9 +29,7 @@ export function assertCompleteToothProsthesisMap(labCase: {
     for (const tooth of teeth) {
       const key = `${link.treatmentDetailId}:${tooth}`;
       if (!prosthesisByKey.has(key)) {
-        throw new BadRequestException(
-          `Each tooth must have a prosthesis type before sending (missing tooth ${tooth})`,
-        );
+        throw new AppException(ErrorCode.TREATMENT_TOOTH_PROSTHESIS_INCOMPLETE, HttpStatus.BAD_REQUEST);
       }
     }
   }