feat: 實現智能例句圖片顯示和AI生成按鈕

- 修改例句圖片邏輯，只顯示已確認存在的圖片
- 為沒有例句圖片的詞彙顯示「新增例句圖」按鈕
- 添加AI生成例句圖片的預留接口
- 提供直觀的視覺提示，區分已有圖片和待生成圖片
- 為下階段AI生成流程做準備

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

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

frontend/app/flashcards/page.tsx

@@ -24,35 +24,29 @@ function FlashcardsContent() {
   const [searchState, searchActions] = useFlashcardSearch(activeTab)
 
   // 例句圖片邏輯
-  const getExampleImage = (word: string): string => {
-    const availableImages = [
-      '/images/examples/bring_up.png',
-      '/images/examples/instinct.png',
-      '/images/examples/warrant.png'
-    ]
-
+  const getExampleImage = (word: string): string | null => {
     const imageMap: {[key: string]: string} = {
       'brought': '/images/examples/bring_up.png',
       'instincts': '/images/examples/instinct.png',
       'warrants': '/images/examples/warrant.png',
-      'hello': '/images/examples/bring_up.png',
-      'beautiful': '/images/examples/instinct.png',
-      'understand': '/images/examples/warrant.png',
-      'elaborate': '/images/examples/bring_up.png',
-      'sophisticated': '/images/examples/instinct.png',
-      'ubiquitous': '/images/examples/warrant.png'
+      'evidence': '/images/examples/bring_up.png',
+      'recovering': '/images/examples/instinct.png'
     }
 
-    // 根據詞彙返回對應圖片，如果沒有則根據字母分配
-    const mappedImage = imageMap[word?.toLowerCase()]
-    if (mappedImage) return mappedImage
+    // 只返回已確認存在的圖片，沒有則返回 null
+    return imageMap[word?.toLowerCase()] || null
+  }
 
-    // 根據首字母分配圖片
-    const firstChar = (word || 'a')[0].toLowerCase()
-    const charCode = firstChar.charCodeAt(0) - 97 // a=0, b=1, c=2...
-    const imageIndex = charCode % availableImages.length
+  // 檢查詞彙是否有例句圖片
+  const hasExampleImage = (word: string): boolean => {
+    return getExampleImage(word) !== null
+  }
 
-    return availableImages[imageIndex]
+  // 處理AI生成例句圖片 (預留接口)
+  const handleGenerateExampleImage = (card: Flashcard) => {
+    console.log('準備為詞彙生成例句圖片:', card.word)
+    // TODO: 下階段實現AI生成流程
+    // router.push(`/generate-image?word=${encodeURIComponent(card.word)}`)
   }
 
   // 初始化數據載入
@@ -258,6 +252,8 @@ function FlashcardsContent() {
           getCEFRColor={getCEFRColor}
           highlightSearchTerm={highlightSearchTerm}
           getExampleImage={getExampleImage}
+          hasExampleImage={hasExampleImage}
+          onGenerateExampleImage={handleGenerateExampleImage}
           router={router}
         />
 
@@ -469,7 +465,9 @@ interface SearchResultsProps {
   onToggleFavorite: (card: Flashcard) => void
   getCEFRColor: (level: string) => string
   highlightSearchTerm: (text: string, term: string) => React.ReactNode
-  getExampleImage: (word: string) => string
+  getExampleImage: (word: string) => string | null
+  hasExampleImage: (word: string) => boolean
+  onGenerateExampleImage: (card: Flashcard) => void
   router: any
 }
 
@@ -482,6 +480,8 @@ function SearchResults({
   getCEFRColor,
   highlightSearchTerm,
   getExampleImage,
+  hasExampleImage,
+  onGenerateExampleImage,
   router
 }: SearchResultsProps) {
   if (searchState.flashcards.length === 0) {
@@ -521,6 +521,8 @@ function SearchResults({
           getCEFRColor={getCEFRColor}
           highlightSearchTerm={highlightSearchTerm}
           getExampleImage={getExampleImage}
+          hasExampleImage={hasExampleImage}
+          onGenerateExampleImage={() => onGenerateExampleImage(card)}
           router={router}
         />
       ))}
@@ -537,11 +539,13 @@ interface FlashcardItemProps {
   onToggleFavorite: () => void
   getCEFRColor: (level: string) => string
   highlightSearchTerm: (text: string, term: string) => React.ReactNode
-  getExampleImage: (word: string) => string
+  getExampleImage: (word: string) => string | null
+  hasExampleImage: (word: string) => boolean
+  onGenerateExampleImage: () => void
   router: any
 }
 
-function FlashcardItem({ card, searchTerm, onEdit, onDelete, onToggleFavorite, getCEFRColor, highlightSearchTerm, getExampleImage, router }: FlashcardItemProps) {
+function FlashcardItem({ card, searchTerm, onEdit, onDelete, onToggleFavorite, getCEFRColor, highlightSearchTerm, getExampleImage, hasExampleImage, onGenerateExampleImage, router }: FlashcardItemProps) {
   return (
     <div className="bg-white border border-gray-200 rounded-lg hover:shadow-md transition-all duration-200 relative">
       <div className="p-4">
@@ -554,25 +558,40 @@ function FlashcardItem({ card, searchTerm, onEdit, onDelete, onToggleFavorite, g
           </div>
 
           <div className="flex items-center gap-4 flex-1">
-            {/* 例句圖片 */}
+            {/* 例句圖片區域 */}
             <div className="w-54 h-36 bg-gray-100 rounded-lg overflow-hidden border border-gray-200 flex items-center justify-center">
-              <img
-                src={getExampleImage(card.word)}
-                alt={`${card.word} example`}
-                className="w-full h-full object-cover"
-                onError={(e) => {
-                  const target = e.target as HTMLImageElement
-                  target.style.display = 'none'
-                  target.parentElement!.innerHTML = `
-                    <div class="text-gray-400 text-xs text-center">
-                      <svg class="w-6 h-6 mx-auto mb-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16l4.586-4.586a2 2 0 012.828 0L16 16m-2-2l1.586-1.586a2 2 0 012.828 0L20 14m-6-6h.01M6 20h12a2 2 0 002-2V6a2 2 0 00-2-2H6a2 2 0 00-2 2v12a2 2 0 002 2z" />
-                      </svg>
-                      例句圖
-                    </div>
-                  `
-                }}
-              />
+              {hasExampleImage(card.word) ? (
+                // 有例句圖片時顯示圖片
+                <img
+                  src={getExampleImage(card.word)!}
+                  alt={`${card.word} example`}
+                  className="w-full h-full object-cover"
+                  onError={(e) => {
+                    const target = e.target as HTMLImageElement
+                    target.style.display = 'none'
+                    target.parentElement!.innerHTML = `
+                      <div class="text-gray-400 text-xs text-center">
+                        <svg class="w-6 h-6 mx-auto mb-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16l4.586-4.586a2 2 0 012.828 0L16 16m-2-2l1.586-1.586a2 2 0 012.828 0L20 14m-6-6h.01M6 20h12a2 2 0 002-2V6a2 2 0 00-2-2H6a2 2 0 00-2 2v12a2 2 0 002 2z" />
+                        </svg>
+                        圖片載入失敗
+                      </div>
+                    `
+                  }}
+                />
+              ) : (
+                // 沒有例句圖片時顯示新增按鈕
+                <button
+                  onClick={onGenerateExampleImage}
+                  className="w-full h-full flex flex-col items-center justify-center text-gray-500 hover:text-blue-600 hover:bg-blue-50 transition-colors group"
+                  title="點擊生成例句圖片"
+                >
+                  <svg className="w-8 h-8 mb-2 group-hover:scale-110 transition-transform" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M12 4v16m8-8H4" />
+                  </svg>
+                  <span className="text-xs font-medium">新增例句圖</span>
+                </button>
+              )}
             </div>
 
             {/* 詞卡信息 */}