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.spec.ts

@@ -1,3 +1,4 @@
+import { AppException } from '../../common/errors';
 import { assertCompleteToothProsthesisMap } from './lab-case-send.validation';
 
 describe('assertCompleteToothProsthesisMap', () => {
@@ -47,6 +48,6 @@ describe('assertCompleteToothProsthesisMap', () => {
           { treatmentDetailId: prosthesisDetailId, tooth: '14', prosthesisTypeCode: 'pfm_crown' },
         ],
       }),
-    ).toThrow('missing tooth 15');
+    ).toThrow(AppException);
   });
 });