feat: iOS 相关改进

1. webview 增加 loading 的功能，同时解决 present 试图，动画不正确的问题
2. 修复 ‘关于手册页面’，GitHub 按钮遮挡状态栏的问题

Signed-off-by: cangzhu <cangzhu@wacai.com>

lib/views/fourth_page/pages.dart

@@ -71,6 +71,7 @@ class Page extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    final paddingTop = MediaQuery.of(context).padding.top;
     return Stack(
         //alignment: const Alignment(1.2, 0.6),
         children: [
@@ -90,7 +91,7 @@ class Page extends StatelessWidget {
           ),
           Positioned(
               right: -5.0,
-              top: 2.0,
+              top: paddingTop + 2.0,
               child: creatButton(context, 'GitHub', Icons.arrow_forward, 'goGithub')
           ),
         ]

lib/views/web_page/web_view_page.dart

@@ -76,6 +76,7 @@ class _WebViewPageState extends State<WebViewPage> {
         withZoom: false,
         withLocalStorage: true,
         withJavascript: true,
+        hidden: true,
       ),
     );
   }