feat: 優化FlipMemoryTest組件用戶體驗

- 將信心等級按鈕移到翻卡外面，裸露在背景上
- 移除按鈕數字，只顯示文字描述且字體更大
- 修復音頻播放按鈕點擊會觸發翻面的問題
- 增加翻卡容器高度至600px提供更多內容空間
- 翻卡背面只保留純學習內容（定義、例句、同義詞）

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

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

frontend/components/review/review-tests/FlipMemoryTest.tsx

@@ -63,11 +63,11 @@ export const FlipMemoryTest: React.FC<FlipMemoryTestProps> = ({
       <div
         className={`card-container ${disabled ? 'pointer-events-none opacity-75' : 'cursor-pointer'}`}
         onClick={handleFlip}
-        style={{ perspective: '1000px', minHeight: '500px' }}
+        style={{ perspective: '1000px', minHeight: '600px' }}
       >
         <div
           className={`card bg-white rounded-xl shadow-lg hover:shadow-xl transition-all duration-600 ${isFlipped ? 'rotate-y-180' : ''}`}
-          style={{ transformStyle: 'preserve-3d', minHeight: '500px' }}
+          style={{ transformStyle: 'preserve-3d', minHeight: '600px' }}
         >
           {/* 正面 */}
           <div
@@ -94,6 +94,7 @@ export const FlipMemoryTest: React.FC<FlipMemoryTestProps> = ({
                       {pronunciation && (
                         <span className="text-lg text-gray-500">{pronunciation}</span>
                       )}
+                      <div onClick={(e) => e.stopPropagation()}>
                         <AudioPlayer text={word} />
                       </div>
                     </div>
@@ -101,6 +102,7 @@ export const FlipMemoryTest: React.FC<FlipMemoryTestProps> = ({
                 </div>
               </div>
             </div>
+          </div>
 
           {/* 背面 */}
           <div
@@ -127,7 +129,7 @@ export const FlipMemoryTest: React.FC<FlipMemoryTestProps> = ({
                   <h3 className="font-semibold text-gray-900 mb-2 text-left">例句</h3>
                   <div className="relative">
                     <p className="text-gray-700 italic mb-2 text-left pr-12">"{example}"</p>
-                    <div className="absolute bottom-0 right-0">
+                    <div className="absolute bottom-0 right-0" onClick={(e) => e.stopPropagation()}>
                       <AudioPlayer text={example} />
                     </div>
                   </div>
@@ -151,34 +153,35 @@ export const FlipMemoryTest: React.FC<FlipMemoryTestProps> = ({
                   </div>
                 )}
 
-                {/* 信心等級評估區 */}
-                <div className="bg-blue-50 rounded-lg p-4">
-                  <h3 className="font-semibold text-blue-900 mb-3 text-left">你對這個單字的熟悉程度：</h3>
-                  <div className="grid grid-cols-5 gap-2">
+              </div>
+            </div>
+          </div>
+        </div>
+      </div>
+
+      {/* 信心等級評估區 - 裸露在背景上 */}
+      {(
+        <div className="mt-6">
+          <div className="grid grid-cols-5 gap-3">
             {[1, 2, 3, 4, 5].map(level => (
               <button
                 key={level}
                 onClick={() => handleConfidenceSelect(level)}
                 disabled={disabled || selectedConfidence !== null}
-                        className={`py-3 px-4 bg-white border border-blue-200 text-blue-700 rounded-lg transition-all text-center ${
+                className={`py-4 px-3 border-2 rounded-lg transition-all text-center font-medium ${
                   selectedConfidence === level
-                            ? 'bg-blue-500 text-white border-blue-500'
-                            : 'hover:bg-blue-100 hover:border-blue-300'
-                        } ${disabled || selectedConfidence !== null ? 'opacity-50 cursor-not-allowed' : ''}`}
+                    ? 'bg-blue-500 text-white border-blue-500 shadow-lg'
+                    : 'bg-white border-gray-300 text-gray-700 hover:bg-blue-50 hover:border-blue-400 shadow-sm'
+                } ${disabled || selectedConfidence !== null ? 'opacity-50 cursor-not-allowed' : 'hover:shadow-md'}`}
               >
-                        <div className="font-bold text-lg">{level}</div>
-                        <div className="text-xs">
+                <div className="text-lg font-semibold">
                   {confidenceLabels[level as keyof typeof confidenceLabels]}
                 </div>
               </button>
             ))}
           </div>
         </div>
-              </div>
-            </div>
-          </div>
-        </div>
-      </div>
+      )}
 
       <style jsx>{`
         .card-container {