feat: 改善詞卡保存用戶反饋體驗

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

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

frontend/app/generate/page.tsx

@@ -218,17 +218,25 @@ function GenerateContent() {
       const response = await flashcardsService.createFlashcard(cardData)
 
       if (response.success) {
-        console.log(`✅ 已將「${word}」保存到詞卡！`)
+        // 顯示成功提示
+        const successMessage = `✅ 已成功將「${word}」保存到詞卡庫！`
+        alert(successMessage)
+        console.log(successMessage)
         return { success: true }
       } else if (response.error && response.error.includes('已存在')) {
-        console.log(`⚠️ 詞卡「${word}」已存在`)
-        return { success: false, error: 'duplicate', message: `詞卡「${word}」已經存在於詞卡庫中` }
+        // 顯示重複提示
+        const duplicateMessage = `⚠️ 詞卡「${word}」已經存在於詞卡庫中`
+        alert(duplicateMessage)
+        console.log(duplicateMessage)
+        return { success: false, error: 'duplicate', message: duplicateMessage }
       } else {
         throw new Error(response.error || '保存失敗')
       }
     } catch (error) {
       console.error('Save word error:', error)
-      return { success: false, error: error instanceof Error ? error.message : '保存失敗' }
+      const errorMessage = error instanceof Error ? error.message : '保存失敗'
+      alert(`❌ 保存詞卡失敗: ${errorMessage}`)
+      return { success: false, error: errorMessage }
     }
   }, [])