feat: #129 Center app bar title

lib/main.dart

@@ -47,24 +47,26 @@ class MyHomePage extends StatefulWidget {
 class _MyHomePageState extends State<MyHomePage> {
   int _currentIndex = 0;
   final List<Widget> _pages = const [Backlog(), TocTree(), Trends()];
-  final _appBarTitles = ['待办', '目录树', '动态'];
+  final _appBarTitles = const ['待办', '目录树', '动态'];
 
   @override
   Widget build(BuildContext context) {
     return Scaffold(
       appBar: AppBar(
         title: Text(_appBarTitles[_currentIndex]),
+        centerTitle: true,
         titleTextStyle: const TextStyle(color: Colors.black),
         backgroundColor: Colors.white,
       ),
       body: _pages[_currentIndex],
       bottomNavigationBar: BottomNavigationBar(
-        items: const [
+        items: [
           BottomNavigationBarItem(
-              icon: Icon(CustomIcons.toDoList), label: "待办"),
+              icon: const Icon(CustomIcons.toDoList), label: _appBarTitles[0]),
           BottomNavigationBarItem(
-              icon: Icon(CustomIcons.catalog), label: "目录树"),
-          BottomNavigationBarItem(icon: Icon(CustomIcons.trends), label: "动态")
+              icon: const Icon(CustomIcons.catalog), label: _appBarTitles[1]),
+          BottomNavigationBarItem(
+              icon: const Icon(CustomIcons.trends), label: _appBarTitles[2])
         ],
         type: BottomNavigationBarType.fixed,
         fixedColor: const Color(0xFFFC6D26),