fix: scroll to top on survey slide transitions and completion

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

frontend/src/pages/SurveyPage.tsx

@@ -199,12 +199,14 @@ export default function SurveyPage() {
       if (!res.ok) {
         if (res.status === 409) {
           setAlreadyCompleted(true)
+          window.scrollTo({ top: 0, behavior: 'smooth' })
           return
         }
         const errData = await res.json().catch(() => null)
         throw new Error(errData?.detail || `Submission failed (${res.status})`)
       }
       setIsComplete(true)
+      window.scrollTo({ top: 0, behavior: 'smooth' })
     } catch (err) {
       setSubmitError(err instanceof Error ? err.message : 'Submission failed. Try copying your responses instead.')
     } finally {