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/i18n/clientTimeZone.ts

@@ -0,0 +1,8 @@
+/** Browser IANA zone (e.g. Asia/Tehran). Used so the UTC server can interpret wall-clock hours. */
+export function getClientTimeZone(): string {
+  try {
+    return Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC';
+  } catch {
+    return 'UTC';
+  }
+}