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;