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.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 {