修复苹果手机上按钮消失问题

- 初始化时校验保存的位置是否在有效范围内
- 确保默认位置始终在边界内 (20%~60%)

pages/index/index.js

@@ -150,18 +150,10 @@ Page({
       
       const systemInfo = wx.getSystemInfoSync();
       const screenHeight = systemInfo.screenHeight * 2;
-      const safeArea = systemInfo.safeArea;
       const btnHeight = 90;
       
-      let minTop, maxTop;
+      const minTop = screenHeight * 0.2;
-      
+      const maxTop = screenHeight * 0.6;
-      if (safeArea && safeArea.top > 20) {
-        minTop = safeArea.top * 2 + 20;
-        maxTop = safeArea.bottom * 2 - btnHeight - 20;
-      } else {
-        minTop = screenHeight * 0.2;
-        maxTop = screenHeight * 0.6;
-      }
       
       if (newTop < minTop) {
         newTop = minTop;
@@ -206,9 +198,13 @@ Page({
     const systemInfo = wx.getSystemInfoSync();
     const screenHeight = systemInfo.screenHeight * 2;
     
-    let defaultTop = screenHeight / 2;
+    let defaultTop = screenHeight * 0.5;
     if (savedBtnTop !== '' && savedBtnTop !== -100) {
-      defaultTop = savedBtnTop;
+      const minTop = screenHeight * 0.2;
+      const maxTop = screenHeight * 0.6;
+      if (savedBtnTop >= minTop && savedBtnTop <= maxTop) {
+        defaultTop = savedBtnTop;
+      }
     }
     
     this.setData({