fix: 修復CEFR顯示錯誤，使用正確的difficultyLevel屬性

- 修復測驗卡片右上角CEFR等級顯示問題
- 將所有currentCard.difficulty改為currentCard.difficultyLevel
- 修復AudioPlayer在p標籤內導致的HTML結構錯誤
- 確保CEFR字符串架構完全正確運作

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

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

frontend/app/learn/page.tsx

@@ -930,7 +930,7 @@ export default function LearnPage() {
                         翻卡記憶
                       </h2>
                       <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                        {currentCard.difficulty}
+                        {currentCard.difficultyLevel}
                       </span>
                     </div>