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/learn/[unitId]/quiz/page.tsx

@@ -73,7 +73,7 @@ export default function QuizPage() {
   }
 
   return (
-    <div className={`min-h-screen flex flex-col ${isDark ? 'bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800' : 'bg-gradient-to-br from-slate-100 via-blue-50 to-cyan-100'}`}>
+    <>
       {/* Header */}
       <div className={`${glassCard} border-0 border-b`}>
         <div className="max-w-2xl mx-auto px-6 py-4 flex items-center justify-between">