fix: 修復ExtendedFlashcard接口類型衝突

- 使用 Omit<Flashcard, 'nextReviewDate'> 避免類型衝突
- 移除未使用的 isA1Learner import
- 修復 nextReviewDate 可選屬性類型錯誤

編譯現在應該無錯誤，頁面可正常顯示

frontend/app/learn/page.tsx

@@ -9,13 +9,13 @@ import LearningComplete from '@/components/LearningComplete'
 import ReviewTypeIndicator from '@/components/review/ReviewTypeIndicator'
 import MasteryIndicator from '@/components/review/MasteryIndicator'
 import { flashcardsService, type Flashcard } from '@/lib/services/flashcards'
-import { calculateCurrentMastery, getReviewTypesByDifficulty, isA1Learner } from '@/lib/utils/masteryCalculator'
+import { calculateCurrentMastery, getReviewTypesByDifficulty } from '@/lib/utils/masteryCalculator'
 
 // 擴展的Flashcard接口，包含智能複習需要的欄位
-interface ExtendedFlashcard extends Flashcard {
+interface ExtendedFlashcard extends Omit<Flashcard, 'nextReviewDate'> {
   userLevel?: number;        // 學習者程度 (1-100)
   wordLevel?: number;        // 詞彙難度 (1-100)
-  nextReviewDate?: string;   // 下次復習日期
+  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