feat: add bottom navigation shell with study/manage/stats/account tabs

lib/app_shell.dart

@@ -0,0 +1,63 @@
+import 'package:flutter/material.dart';
+
+import 'pages/account_page.dart';
+import 'pages/manage_page.dart';
+import 'pages/stats_page.dart';
+import 'pages/study_page.dart';
+
+/// 应用外壳：底部四个主入口。
+///
+/// 用 [IndexedStack] 而不是直接切换 widget，是为了在切页时保留各页面的
+/// 状态（滚动位置、输入内容、进行中的学习会话）。
+class AppShell extends StatefulWidget {
+  const AppShell({super.key});
+
+  @override
+  State<AppShell> createState() => _AppShellState();
+}
+
+class _AppShellState extends State<AppShell> {
+  int _selectedIndex = 0;
+
+  static const List<Widget> _pages = <Widget>[
+    StudyPage(),
+    ManagePage(),
+    StatsPage(),
+    AccountPage(),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: IndexedStack(index: _selectedIndex, children: _pages),
+      bottomNavigationBar: NavigationBar(
+        selectedIndex: _selectedIndex,
+        onDestinationSelected: (int index) {
+          setState(() => _selectedIndex = index);
+        },
+        destinations: const <NavigationDestination>[
+          NavigationDestination(
+            icon: Icon(Icons.school_outlined),
+            selectedIcon: Icon(Icons.school),
+            label: '学习',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.library_books_outlined),
+            selectedIcon: Icon(Icons.library_books),
+            label: '管理',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.insights_outlined),
+            selectedIcon: Icon(Icons.insights),
+            label: '统计',
+          ),
+          NavigationDestination(
+            icon: Icon(Icons.person_outline),
+            selectedIcon: Icon(Icons.person),
+            label: '账户',
+          ),
+        ],
+      ),
+    );
+  }
+}