fix: 修復詞卡頁面 TypeScript 錯誤並清理冗餘代碼

🔧 TypeScript 錯誤修復：
- 移除循環引用：刪除 `type Flashcard = Flashcard` 重複定義
- 清理 import 衝突：直接使用從 flashcards.ts 導入的 Flashcard 型別
- 移除未使用變數：刪除 mockFlashcards 假資料定義
- 清理冗餘型別：移除不需要的 CardSet 型別定義

🧹 代碼清理：
- 完全移除假資料依賴，現在純粹使用真實 API
- 簡化型別結構，避免不必要的型別重複定義
- 提升代碼可維護性和類型安全性

現在詞卡頁面沒有 TypeScript 錯誤，所有功能正常運作。

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

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

frontend/app/flashcards/page.tsx

@@ -9,15 +9,7 @@ import { useToast } from '@/components/Toast'
 // import { flashcardsService, type CardSet, type Flashcard } from '@/lib/services/flashcards'
 import { flashcardsService, type Flashcard } from '@/lib/services/flashcards'
 
-// 暫時為了兼容性定義 CardSet 類型
-type CardSet = {
-  id: string;
-  name: string;
-  color: string;
-}
-
-// 使用簡化的 Flashcard 類型
-type Flashcard = Flashcard
+// 移除不需要的型別定義，直接使用 import 的 Flashcard 型別
 import { useRouter } from 'next/navigation'
 
 function FlashcardsContent() {
@@ -77,15 +69,7 @@ function FlashcardsContent() {
   const [showForm, setShowForm] = useState(false)
   const [editingCard, setEditingCard] = useState<Flashcard | null>(null)
 
-  // 添加假資料用於展示CEFR效果 (更新為 Flashcard 格式)
-  const mockFlashcards: Flashcard[] = [
-    { id: 'mock1', word: 'hello', translation: '你好', partOfSpeech: 'interjection', pronunciation: '/həˈloʊ/', masteryLevel: 95, timesReviewed: 15, isFavorite: true, nextReviewDate: '2025-09-21', difficultyLevel: 'A1', definition: 'A greeting word', example: 'Hello, how are you?', createdAt: '2025-09-17', updatedAt: '2025-09-17' },
-    { id: 'mock2', word: 'beautiful', translation: '美麗的', partOfSpeech: 'adjective', pronunciation: '/ˈbjuːtɪfəl/', masteryLevel: 78, timesReviewed: 8, isFavorite: false, nextReviewDate: '2025-09-22', difficultyLevel: 'A2', definition: 'Pleasing to look at', example: 'The beautiful sunset', createdAt: '2025-09-16', updatedAt: '2025-09-16' },
-    { id: 'mock3', word: 'understand', translation: '理解', partOfSpeech: 'verb', pronunciation: '/ˌʌndərˈstænd/', masteryLevel: 65, timesReviewed: 12, isFavorite: true, nextReviewDate: '2025-09-20', difficultyLevel: 'B1', definition: 'To comprehend', example: 'I understand the concept', createdAt: '2025-09-15', updatedAt: '2025-09-15' },
-    { id: 'mock4', word: 'elaborate', translation: '詳細說明', partOfSpeech: 'verb', pronunciation: '/ɪˈlæbərət/', masteryLevel: 45, timesReviewed: 5, isFavorite: false, nextReviewDate: '2025-09-19', difficultyLevel: 'B2', definition: 'To explain in detail', example: 'Please elaborate on your idea', createdAt: '2025-09-14', updatedAt: '2025-09-14' },
-    { id: 'mock5', word: 'sophisticated', translation: '精密的', partOfSpeech: 'adjective', pronunciation: '/səˈfɪstɪkeɪtɪd/', masteryLevel: 30, timesReviewed: 3, isFavorite: true, nextReviewDate: '2025-09-18', difficultyLevel: 'C1', definition: 'Highly developed', example: 'A sophisticated system', createdAt: '2025-09-13', updatedAt: '2025-09-13' },
-    { id: 'mock6', word: 'ubiquitous', translation: '無處不在的', partOfSpeech: 'adjective', pronunciation: '/juːˈbɪkwɪtəs/', masteryLevel: 15, timesReviewed: 1, isFavorite: false, nextReviewDate: '2025-09-17', difficultyLevel: 'C2', definition: 'Present everywhere', example: 'Smartphones are ubiquitous', createdAt: '2025-09-12', updatedAt: '2025-09-12' }
-  ]
+  // 移除假資料，現在完全使用真實 API 資料
 
   // 載入總數統計
   const loadTotalCounts = async () => {
@@ -233,7 +217,6 @@ function FlashcardsContent() {
 
 
   // 由於後端已處理篩選，直接使用 API 回傳的結果
-  const allCards = [...flashcards, ...mockFlashcards] // 保留模擬資料用於展示
   const filteredCards = flashcards // 直接使用從 API 取得的已篩選結果
 
   // 清除所有篩選