Add central layout.tsx for /learn/* and /parent/* routes

Next.js route-level layouts provide Sidebar + gradient background
automatically for all sub-pages. Individual pages no longer need
their own wrapper divs or Sidebar imports.

- learn/layout.tsx: Sidebar + purple gradient for all learning pages
- parent/layout.tsx: Same for all parent portal pages
- LearnLayout.tsx: Reusable component for other pages
- Fixed broken <LearnLayout>}> artifacts from previous refactoring
- Removed duplicate Sidebar/wrapper code from 9 sub-pages

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

studio-v2/app/onboarding/page.tsx

@@ -5,6 +5,7 @@ import { useRouter } from 'next/navigation'
 import { useTheme } from '@/lib/ThemeContext'
 import { useLanguage } from '@/lib/LanguageContext'
 import type { Language } from '@/lib/i18n'
+import { LearnLayout } from '@/components/learn/LearnLayout'
 
 interface LangOption {
   code: string