fix: scroll to top after survey slide transition renders

Use requestAnimationFrame to defer scrollTo until after React
renders the new slide content, preventing the browser from
staying at the bottom of the page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/SurveyPage.tsx

@@ -188,7 +188,9 @@ export default function SurveyPage() {
 
   const goSlide = (idx: number) => {
     setCurrentSlide(idx)
-    window.scrollTo({ top: 0, behavior: 'smooth' })
+    requestAnimationFrame(() => {
+      window.scrollTo({ top: 0, behavior: 'smooth' })
+    })
   }
 
   const handleSubmit = async () => {