feat: 實現用戶隔離和重複保存告警機制

重大改進：
- 修復用戶隔離快取機制，使用用戶 ID 區分快取 key
- 添加重複保存告警，防止同一次分析中重複保存相同詞彙
- 實現 JWT token 解析獲取用戶 ID，確保資料隔離
- 添加 24 小時快取過期機制

用戶體驗優化：
- 已保存詞彙顯示綠色勾號視覺標記
- 重複保存時提供確認對話框和清楚說明
- 新分析時自動重置已保存詞彙追蹤
- 完整的快取驗證和安全檢查

隱私保護：
- 不同用戶的分析結果完全隔離
- 換帳號後無法看到其他用戶的快取內容

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

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

frontend/app/generate/page.tsx

@@ -48,32 +48,82 @@ function GenerateContent() {
   // UX 改善：追蹤分析狀態，避免輸入和結果不匹配
   const [lastAnalyzedText, setLastAnalyzedText] = useState('')
 
-  // localStorage 快取函數
+  // 追蹤本次會話已保存的詞彙（防重複保存）
-  const saveAnalysisToCache = useCallback((cacheData: any) => {
+  const [savedWordsInSession, setSavedWordsInSession] = useState<Set<string>>(new Set())
-    try {
+
-      localStorage.setItem('generate_analysis_cache', JSON.stringify(cacheData))
+  // 獲取當前用戶 ID 用於快取隔離
-    } catch (error) {
+  const getCurrentUserId = useCallback(() => {
-      console.warn('無法保存分析快取:', error)
+    const token = typeof window !== 'undefined' ? localStorage.getItem('auth_token') : null
-    }
+    if (!token) return null
-  }, [])
 
-  const loadAnalysisFromCache = useCallback(() => {
     try {
-      const cached = localStorage.getItem('generate_analysis_cache')
+      // 解析 JWT token 獲取用戶 ID
-      return cached ? JSON.parse(cached) : null
+      const payload = JSON.parse(atob(token.split('.')[1]))
-    } catch (error) {
+      return payload.nameid || payload.sub || null
-      console.warn('無法載入分析快取:', error)
+    } catch {
       return null
     }
   }, [])
 
+  // localStorage 快取函數（用戶隔離）
+  const saveAnalysisToCache = useCallback((cacheData: any) => {
+    try {
+      const userId = getCurrentUserId()
+      if (!userId) return
+
+      const cacheKey = `generate_analysis_cache_${userId}`
+      localStorage.setItem(cacheKey, JSON.stringify({
+        ...cacheData,
+        userId,
+        timestamp: Date.now()
+      }))
+    } catch (error) {
+      console.warn('無法保存分析快取:', error)
+    }
+  }, [getCurrentUserId])
+
+  const loadAnalysisFromCache = useCallback(() => {
+    try {
+      const userId = getCurrentUserId()
+      if (!userId) return null
+
+      const cacheKey = `generate_analysis_cache_${userId}`
+      const cached = localStorage.getItem(cacheKey)
+      if (!cached) return null
+
+      const cacheData = JSON.parse(cached)
+
+      // 驗證快取是否屬於當前用戶
+      if (cacheData.userId !== userId) {
+        localStorage.removeItem(cacheKey)
+        return null
+      }
+
+      // 檢查快取是否過期（24小時）
+      const isExpired = Date.now() - cacheData.timestamp > 24 * 60 * 60 * 1000
+      if (isExpired) {
+        localStorage.removeItem(cacheKey)
+        return null
+      }
+
+      return cacheData
+    } catch (error) {
+      console.warn('無法載入分析快取:', error)
+      return null
+    }
+  }, [getCurrentUserId])
+
   const clearAnalysisCache = useCallback(() => {
     try {
-      localStorage.removeItem('generate_analysis_cache')
+      const userId = getCurrentUserId()
+      if (!userId) return
+
+      const cacheKey = `generate_analysis_cache_${userId}`
+      localStorage.removeItem(cacheKey)
     } catch (error) {
       console.warn('無法清除分析快取:', error)
     }
-  }, [])
+  }, [getCurrentUserId])
 
   // 組件載入時恢復快取的分析結果
   useEffect(() => {
@@ -84,6 +134,12 @@ function GenerateContent() {
       setSentenceAnalysis(cached.sentenceAnalysis || null)
       setSentenceMeaning(cached.sentenceMeaning || '')
       setGrammarCorrection(cached.grammarCorrection || null)
+
+      // 恢復已保存詞彙的狀態
+      if (cached.savedWordsInSession) {
+        setSavedWordsInSession(new Set(cached.savedWordsInSession))
+      }
+
       console.log('✅ 已恢復快取的分析結果')
     }
   }, [loadAnalysisFromCache])
@@ -94,6 +150,9 @@ function GenerateContent() {
     // 清除舊的分析快取
     clearAnalysisCache()
 
+    // 重置本次會話的已保存詞彙追蹤
+    setSavedWordsInSession(new Set())
+
     setIsAnalyzing(true)
 
     try {
@@ -154,7 +213,7 @@ function GenerateContent() {
         })
       }
 
-      // 保存分析結果到快取
+      // 保存分析結果到快取（包含已保存詞彙狀態）
       const cacheData = {
         textInput,
         sentenceAnalysis: analysisData,
@@ -165,7 +224,8 @@ function GenerateContent() {
           correctedText: apiData.grammarCorrection.correctedText || textInput,
           corrections: apiData.grammarCorrection.corrections || [],
           confidenceScore: apiData.grammarCorrection.confidenceScore || 1.0
-        } : null
+        } : null,
+        savedWordsInSession: Array.from(savedWordsInSession)
       }
       saveAnalysisToCache(cacheData)
 
@@ -222,9 +282,21 @@ function GenerateContent() {
     }
   }, [sentenceAnalysis, userLevel])
 
-  // 保存詞彙
+  // 保存詞彙（帶重複檢查）
   const handleSaveWord = useCallback(async (word: string, analysis: any) => {
     try {
+      // 檢查是否已在本次會話中保存過
+      if (savedWordsInSession.has(word.toLowerCase())) {
+        const confirmSave = window.confirm(
+          `詞彙「${word}」已在本次分析中保存過，確定要重複保存嗎？\n\n重複保存會創建完全相同的詞卡。`
+        )
+
+        if (!confirmSave) {
+          toast.info(`已取消保存「${word}」`)
+          return { success: false, cancelled: true }
+        }
+      }
+
       const cardData = {
         word: word,
         translation: analysis.translation || '',
@@ -239,6 +311,9 @@ function GenerateContent() {
 
       const response = await flashcardsService.createFlashcard(cardData)
       if (response.success) {
+        // 標記為已保存
+        setSavedWordsInSession(prev => new Set([...prev, word.toLowerCase()]))
+
         toast.success(`已成功將「${word}」保存到詞卡庫！`)
         return { success: true }
       } else {
@@ -249,7 +324,7 @@ function GenerateContent() {
       toast.error(`保存詞卡失敗: ${errorMessage}`)
       return { success: false, error: errorMessage }
     }
-  }, [toast])
+  }, [toast, savedWordsInSession])
 
   return (
     <div className="min-h-screen bg-gradient-to-br from-blue-50 to-indigo-100">
@@ -400,15 +475,25 @@ function GenerateContent() {
                         return <span key={index} className="text-gray-900">{token}</span>
                       }
 
+                      const isSaved = savedWordsInSession.has(cleanToken.toLowerCase())
+
                       return (
                         <span key={index}>
                           <span
-                            className={getWordClass(cleanToken, analysis, userLevel)}
+                            className={`${getWordClass(cleanToken, analysis, userLevel)} ${
+                              isSaved ? 'relative' : ''
+                            }`}
                             onClick={() => setSelectedWord(cleanToken)}
                             role="button"
                             tabIndex={0}
                           >
                             {token}
+                            {isSaved && (
+                              <span className="absolute -top-1 -right-1 w-3 h-3 bg-green-500 rounded-full border border-white"
+                                    title="已保存到詞卡庫">
+                                <span className="absolute inset-0 flex items-center justify-center text-white text-xs">✓</span>
+                              </span>
+                            )}
                           </span>
                         </span>
                       )