fix(wish-tree): 修复切换树时导航栏消失问题

- Canvas添加z-index: 1，确保不覆盖导航栏
- top-bar添加pointer-events: none，子元素恢复auto
- WXML添加trees.length保护，防止空数组访问
- switchTree添加空数组保护

mini/pages/wish-tree/wish-tree.js

@@ -939,6 +939,8 @@ Page({
     const direction = e.currentTarget.dataset.direction;
     const { trees, currentTreeIndex } = this.data;
     
+    if (!trees.length) return;
+    
     let newIndex = currentTreeIndex;
     if (direction === 'prev') {
       newIndex = (currentTreeIndex - 1 + trees.length) % trees.length;

mini/pages/wish-tree/wish-tree.wxml

@@ -13,7 +13,7 @@
       <view class="switch-btn" bindtap="switchTree" data-direction="prev">
         <text class="switch-icon">‹</text>
       </view>
-      <view class="tree-name">{{trees[currentTreeIndex].name || '许愿树'}}</view>
+      <view class="tree-name">{{trees.length > 0 ? trees[currentTreeIndex].name : '许愿树'}}</view>
       <view class="switch-btn" bindtap="switchTree" data-direction="next">
         <text class="switch-icon">›</text>
       </view>

mini/pages/wish-tree/wish-tree.wxss

@@ -13,6 +13,7 @@
   left: 0;
   width: 100%;
   height: 100%;
+  z-index: 1;
 }
 
 /* 顶部导航 */
@@ -26,6 +27,11 @@
   justify-content: space-between;
   align-items: center;
   z-index: 100;
+  pointer-events: none;
+}
+
+.top-bar > * {
+  pointer-events: auto;
 }
 
 .tree-switcher {