feat: 为 WechatModal 组件新增淡出动画效果，并优化进入和离开动画的实现

src/components/WechatModal.vue

@@ -378,6 +378,11 @@ onUnmounted(() => {
   to { opacity: 1; }
 }
 
+@keyframes fadeOut {
+  from { opacity: 1; }
+  to { opacity: 0; }
+}
+
 @keyframes scaleUp {
   from {
     transform: scale(0.8) translateY(20px);
@@ -389,17 +394,32 @@ onUnmounted(() => {
   }
 }
 
+@keyframes scaleDown {
+  from {
+    transform: scale(1) translateY(0);
+    opacity: 1;
+  }
+  to {
+    transform: scale(0.8) translateY(20px);
+    opacity: 0;
+  }
+}
+
+/* 过渡动画 */
+.wechat-modal-enter-active {
+  animation: fadeIn 0.3s ease;
+}
+
+.wechat-modal-leave-active {
+  animation: fadeOut 0.3s ease;
+}
+
 .wechat-modal-enter-active .wechat-modal-container {
   animation: scaleUp 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.1);
 }
 
 .wechat-modal-leave-active .wechat-modal-container {
-  animation: scaleUp 0.2s reverse;
+  animation: scaleDown 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.1);
-}
-
-.wechat-modal-enter-active .wechat-modal-container,
-.wechat-modal-leave-active .wechat-modal-container {
-  transition: all 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.1);
 }
 
 /* 响应式布局 */
@@ -429,6 +449,10 @@ onUnmounted(() => {
 
 /* 减少运动偏好设置 */
 @media (prefers-reduced-motion: reduce) {
+  .wechat-modal-enter-active,
+  .wechat-modal-leave-active,
+  .wechat-modal-enter-active .wechat-modal-container,
+  .wechat-modal-leave-active .wechat-modal-container,
   .wechat-modal-container,
   .wechat-modal-mask,
   .qr-code-image,