feat(wizard): renderer pregunta-por-pantalla con layout estable [4D]

components/wizard-form.tsx

@@ -1,5 +1,5 @@
 'use client'
-import { useEffect, useState } from 'react'
+import { useEffect, useLayoutEffect, useRef, useState } from 'react'
 import type { BrandConfig, Survey } from '@/lib/types'
 import type { AnswerValue, FormAnswers, FormSchema, Screen } from '@/lib/form-engine.types'
 import {
@@ -52,6 +52,8 @@ export function WizardForm({ survey, schema }: WizardFormProps) {
   const [errors, setErrors] = useState<Record<string, string>>({})
   const [hydrated, setHydrated] = useState(false)
   const [a11yAnnounce, setA11yAnnounce] = useState('')
+  const answerAreaRef = useRef<HTMLDivElement>(null)
+  const [hasOverflow, setHasOverflow] = useState(false)
 
   useEffect(() => {
     const draft = loadDraft(survey.id)
@@ -78,6 +80,24 @@ export function WizardForm({ survey, schema }: WizardFormProps) {
     else setA11yAnnounce('Fin del formulario')
   }, [screen?.id])
 
+  // El fade de wizard-answer-area solo debe aparecer si hay overflow real —
+  // CSS no puede expresar esa condición (no hay selector para "este elemento
+  // scrollea"), así que se mide el DOM. Se remide al cambiar de pantalla
+  // (cambia todo el contenido) y al resize (puede cambiar cuántas opciones entran).
+  useLayoutEffect(() => {
+    const el = answerAreaRef.current
+    setHasOverflow(el ? el.scrollHeight > el.clientHeight : false)
+  }, [screen?.id])
+
+  useEffect(() => {
+    function recalc() {
+      const el = answerAreaRef.current
+      setHasOverflow(el ? el.scrollHeight > el.clientHeight : false)
+    }
+    window.addEventListener('resize', recalc)
+    return () => window.removeEventListener('resize', recalc)
+  }, [])
+
   if (stage === 'consent') {
     return (
       <ConsentScreen
@@ -193,7 +213,10 @@ export function WizardForm({ survey, schema }: WizardFormProps) {
               <h2 className="wizard-prompt-text">{screen.question.prompt}</h2>
               {screen.question.hint && <p className="wizard-prompt-hint">{screen.question.hint}</p>}
             </div>
-            <div className="wizard-answer-area">
+            <div
+              ref={answerAreaRef}
+              className={`wizard-answer-area${hasOverflow ? ' has-overflow' : ''}`}
+            >
               <QuestionRenderer
                 question={screen.question}
                 value={currentValue}