Fix booking flow scroll position on mobile step changes.

Scroll to the top whenever the booking step changes so users are not left at the bottom of the page after submitting the form.

frontend/src/app/(public)/book/[eventId]/page.tsx

@@ -228,6 +228,12 @@ export default function BookingPage() {
     }
   }, [agreedToTerms, termsError]);
 
+  // Scroll to top when moving between booking steps (esp. mobile, where
+  // the submit button sits at the bottom of a long form)
+  useEffect(() => {
+    window.scrollTo({ top: 0, behavior: 'smooth' });
+  }, [step]);
+
   const formatDate = (dateStr: string) => formatDateLong(dateStr, locale as 'en' | 'es');
   const fmtTime = (dateStr: string) => formatTime(dateStr, locale as 'en' | 'es');