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]/flashcards/page.tsx

@@ -101,7 +101,7 @@ export default function FlashcardsPage() {
   }
 
   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">

studio-v2/app/learn/[unitId]/listen/page.tsx

@@ -74,7 +74,7 @@ export default function ListenPage() {
   const currentItem = items[currentIndex]
 
   return (
-    <div className={`min-h-screen flex flex-col ${isDark ? 'bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800' : 'bg-gradient-to-br from-slate-100 via-green-50 to-emerald-100'}`}>
+    <>
       {/* Header */}
       <div className={`${glassCard} border-0 border-b`}>
         <div className="max-w-2xl mx-auto px-6 py-4 flex items-center justify-between">

studio-v2/app/learn/[unitId]/match/page.tsx

@@ -95,7 +95,7 @@ export default function MatchPage() {
   const matchedTotal = round * 6 + matched.size
 
   return (
-    <div className={`min-h-screen flex flex-col ${isDark ? 'bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800' : 'bg-gradient-to-br from-slate-100 via-indigo-50 to-violet-100'}`}>
+    <>
       {/* Header */}
       <div className={`${glassCard} border-0 border-b`}>
         <div className="max-w-2xl mx-auto px-6 py-4 flex items-center justify-between">

studio-v2/app/learn/[unitId]/pronounce/page.tsx

@@ -67,7 +67,7 @@ export default function PronouncePage() {
   }
 
   return (
-    <div className={`min-h-screen flex flex-col ${isDark ? 'bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800' : 'bg-gradient-to-br from-slate-100 via-rose-50 to-red-100'}`}>
+    <>
       {/* Header */}
       <div className={`${glassCard} border-0 border-b`}>
         <div className="max-w-2xl mx-auto px-6 py-4 flex items-center justify-between">

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">

studio-v2/app/learn/[unitId]/story/page.tsx

@@ -65,7 +65,7 @@ export default function StoryPage() {
   }
 
   return (
-    <div className={`min-h-screen flex flex-col ${isDark ? 'bg-gradient-to-br from-indigo-900 via-purple-900 to-pink-800' : 'bg-gradient-to-br from-slate-100 via-amber-50 to-orange-100'}`}>
+    <>
       {/* Header */}
       <div className={`${glassCard} border-0 border-b`}>
         <div className="max-w-2xl mx-auto px-6 py-4 flex items-center justify-between">

studio-v2/app/learn/[unitId]/type/page.tsx

@@ -94,7 +94,7 @@ export default function TypePage() {
   }
 
   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">