fix: 修復前端TypeScript類型錯誤，完成重命名統一

修復重命名過程中遺留的類型定義問題：
- 更新所有SimpleFlashcard類型引用為Flashcard
- 清理Next.js webpack快取解決編譯錯誤
- 確保前端服務正確連接到新的/api/flashcards端點

✅ 前端重新編譯成功，所有類型錯誤已解決
✅ 智能詞卡生成和保存功能現已完全正常

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

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

frontend/lib/services/flashcards.ts

@@ -57,7 +57,7 @@ class FlashcardsService {
   }
 
   // 簡化的詞卡方法
-  async getFlashcards(search?: string, favoritesOnly: boolean = false): Promise<ApiResponse<{ flashcards: SimpleFlashcard[], count: number }>> {
+  async getFlashcards(search?: string, favoritesOnly: boolean = false): Promise<ApiResponse<{ flashcards: Flashcard[], count: number }>> {
     try {
       const params = new URLSearchParams();
       if (search) params.append('search', search);
@@ -66,7 +66,7 @@ class FlashcardsService {
       const queryString = params.toString();
       const endpoint = `/flashcards${queryString ? `?${queryString}` : ''}`;
 
-      return await this.makeRequest<ApiResponse<{ flashcards: SimpleFlashcard[], count: number }>>(endpoint);
+      return await this.makeRequest<ApiResponse<{ flashcards: Flashcard[], count: number }>>(endpoint);
     } catch (error) {
       return {
         success: false,
@@ -75,7 +75,7 @@ class FlashcardsService {
     }
   }
 
-  async createFlashcard(data: CreateSimpleFlashcardRequest): Promise<ApiResponse<SimpleFlashcard>> {
+  async createFlashcard(data: CreateFlashcardRequest): Promise<ApiResponse<Flashcard>> {
     try {
       return await this.makeRequest<ApiResponse<Flashcard>>('/flashcards', {
         method: 'POST',
@@ -102,7 +102,7 @@ class FlashcardsService {
     }
   }
 
-  async getFlashcard(id: string): Promise<ApiResponse<SimpleFlashcard>> {
+  async getFlashcard(id: string): Promise<ApiResponse<Flashcard>> {
     try {
       return await this.makeRequest<ApiResponse<Flashcard>>(`/flashcards/${id}`);
     } catch (error) {
@@ -113,7 +113,7 @@ class FlashcardsService {
     }
   }
 
-  async updateFlashcard(id: string, data: CreateSimpleFlashcardRequest): Promise<ApiResponse<SimpleFlashcard>> {
+  async updateFlashcard(id: string, data: CreateFlashcardRequest): Promise<ApiResponse<Flashcard>> {
     try {
       return await this.makeRequest<ApiResponse<Flashcard>>(`/flashcards/${id}`, {
         method: 'PUT',