fix: 統一學習頁面導航欄設計，移除結束學習按鈕

- 移除Navigation組件的showExitLearning和onExitLearning props
- 學習頁面右上角菜單現在與其他頁面保持一致
- 用戶可以通過標準導航菜單正常切換頁面
- 提升界面一致性和用戶體驗

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/app/learn/page.tsx

@@ -647,10 +647,7 @@ export default function LearnPage() {
   if (showNoDueCards) {
     return (
       <div className="min-h-screen bg-gradient-to-br from-blue-50 to-indigo-100">
-        <Navigation
-          showExitLearning={true}
-          onExitLearning={() => router.push('/dashboard')}
-        />
+        <Navigation />
         <div className="max-w-4xl mx-auto px-4 py-8 flex items-center justify-center min-h-[calc(100vh-80px)]">
           <div className="bg-white rounded-xl p-8 max-w-md w-full text-center shadow-lg">
             <div className="text-6xl mb-4">📚</div>
@@ -711,10 +708,7 @@ export default function LearnPage() {
   return (
     <div className="min-h-screen bg-gradient-to-br from-blue-50 to-indigo-100">
       {/* Navigation */}
-      <Navigation
-        showExitLearning={true}
-        onExitLearning={() => router.push('/dashboard')}
-      />
+      <Navigation />
 
       <div className="max-w-4xl mx-auto px-4 py-8">
         {/* Progress Bar */}