Fix: Cast language selection to Language type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/onboarding/page.tsx

@@ -4,6 +4,7 @@ import React, { useState, useEffect } from 'react'
 import { useRouter } from 'next/navigation'
 import { useTheme } from '@/lib/ThemeContext'
 import { useLanguage } from '@/lib/LanguageContext'
+import type { Language } from '@/lib/i18n'
 
 interface LangOption {
   code: string
@@ -63,7 +64,7 @@ export default function OnboardingPage() {
 
     // Save locally + set UI language
     localStorage.setItem(STORAGE_KEY, selected)
-    setLanguage(selected)
+    setLanguage(selected as Language)
 
     // Navigate
     if (role === 'parent') {