fix: 修復卡片右上角CEFR顯示空白問題

- 移除ExtendedFlashcard接口中冗余的difficulty屬性
- 所有CEFR顯示統一使用difficultyLevel屬性
- 確保卡片右上角正確顯示CEFR等級 (A1, A2, B1, etc.)
- 完善接口註釋，避免屬性混淆

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

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

frontend/app/learn/page.tsx

@@ -13,17 +13,17 @@ import { calculateCurrentMastery, getReviewTypesByDifficulty } from '@/lib/utils
 
 // 擴展的Flashcard接口，包含智能複習需要的欄位
 interface ExtendedFlashcard extends Omit<Flashcard, 'nextReviewDate'> {
-  userLevel?: number;        // 學習者程度 (1-100)
-  wordLevel?: number;        // 詞彙難度 (1-100)
+  userLevel?: number;        // 學習者程度 (1-100) - 向後兼容
+  wordLevel?: number;        // 詞彙難度 (1-100) - 向後兼容
   nextReviewDate?: string;   // 下次復習日期 (可選)
   currentInterval?: number;  // 當前間隔天數
   isOverdue?: boolean;       // 是否逾期
   overdueDays?: number;      // 逾期天數
   baseMasteryLevel?: number; // 基礎熟悉度
   lastReviewDate?: string;   // 最後復習日期
-  synonyms?: string[];       // 同義詞 (暫時保留mock格式)
-  difficulty?: string;       // CEFR等級 (暫時保留mock格式)
-  exampleImage?: string;     // 例句圖片 (暫時保留mock格式)
+  synonyms?: string[];       // 同義詞 (向後兼容)
+  exampleImage?: string;     // 例句圖片 (向後兼容)
+  // 注意：difficultyLevel已在基礎Flashcard接口中定義
 }
 
 export default function LearnPage() {
@@ -1042,7 +1042,7 @@ export default function LearnPage() {
                   詞彙選擇
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>
 
@@ -1155,7 +1155,7 @@ export default function LearnPage() {
                   例句填空
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>
 
@@ -1324,7 +1324,7 @@ export default function LearnPage() {
                   詞彙聽力 (暫時不上線)
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>
 
@@ -1435,7 +1435,7 @@ export default function LearnPage() {
                   例句口說
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>
 
@@ -1504,7 +1504,7 @@ export default function LearnPage() {
                   例句聽力
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>
 
@@ -1613,7 +1613,7 @@ export default function LearnPage() {
                   例句重組
                 </h2>
                 <span className="bg-blue-100 text-blue-800 text-xs px-2 py-1 rounded-full">
-                  {currentCard.difficulty}
+                  {currentCard.difficultyLevel}
                 </span>
               </div>