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>

frontend/src/lib/api/staff.ts

@@ -1,4 +1,5 @@
 import { apiClient } from './client';
+import { getClientTimeZone } from '@/lib/i18n/clientTimeZone';
 
 export interface StaffMemberDto {
   id: string;
@@ -142,7 +143,10 @@ export const staffApi = {
       blocks: { dayOfWeek: number; startMinute: number; endMinute: number; sortOrder?: number }[];
     },
   ): Promise<{ success: boolean; message: string }> => {
-    const response = await apiClient.put(`/staff/members/${membershipId}/working-hours`, body);
+    const response = await apiClient.put(`/staff/members/${membershipId}/working-hours`, {
+      ...body,
+      timeZone: getClientTimeZone(),
+    });
     return response.data;
   },
 };