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',
+          ),
+        ],
+      ),
+    );
+  }
+}

lib/main.dart

@@ -1,121 +1,35 @@
 import 'package:flutter/material.dart';
 
+import 'app_shell.dart';
+
+/// Dart 层的应用名。各平台原生的应用名（窗口标题、桌面图标）由各自的配置
+/// 文件提供：android 的 `app_name` 字符串资源、ios 的 `CFBundleDisplayName`、
+/// linux/windows 的 `APPLICATION_TITLE`，文本与这里保持一致。
+const String appName = 'KiriMemo';
+
 void main() {
-  runApp(const MyApp());
+  runApp(const KiriMemoApp());
 }
 
-class MyApp extends StatelessWidget {
-  const MyApp({super.key});
+class KiriMemoApp extends StatelessWidget {
+  const KiriMemoApp({super.key});
 
-  // This widget is the root of your application.
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-      title: 'Flutter Demo',
-      theme: ThemeData(
-        // This is the theme of your application.
-        //
-        // TRY THIS: Try running your application with "flutter run". You'll see
-        // the application has a purple toolbar. Then, without quitting the app,
-        // try changing the seedColor in the colorScheme below to Colors.green
-        // and then invoke "hot reload" (save your changes or press the "hot
-        // reload" button in a Flutter-supported IDE, or press "r" if you used
-        // the command line to start the app).
-        //
-        // Notice that the counter didn't reset back to zero; the application
-        // state is not lost during the reload. To reset the state, use hot
-        // restart instead.
-        //
-        // This works for code too, not just values: Most code changes can be
-        // tested with just a hot reload.
-        colorScheme: .fromSeed(seedColor: Colors.deepPurple),
-      ),
-      home: const MyHomePage(title: 'Flutter Demo Home Page'),
+      title: appName,
+      debugShowCheckedModeBanner: false,
+      theme: _theme(Brightness.light),
+      darkTheme: _theme(Brightness.dark),
+      home: const AppShell(),
     );
   }
-}
 
-class MyHomePage extends StatefulWidget {
-  const MyHomePage({super.key, required this.title});
-
-  // This widget is the home page of your application. It is stateful, meaning
-  // that it has a State object (defined below) that contains fields that affect
-  // how it looks.
-
-  // This class is the configuration for the state. It holds the values (in this
-  // case the title) provided by the parent (in this case the App widget) and
-  // used by the build method of the State. Fields in a Widget subclass are
-  // always marked "final".
-
-  final String title;
-
-  @override
-  State<MyHomePage> createState() => _MyHomePageState();
-}
-
-class _MyHomePageState extends State<MyHomePage> {
-  int _counter = 0;
-
-  void _incrementCounter() {
-    setState(() {
-      // This call to setState tells the Flutter framework that something has
-      // changed in this State, which causes it to rerun the build method below
-      // so that the display can reflect the updated values. If we changed
-      // _counter without calling setState(), then the build method would not be
-      // called again, and so nothing would appear to happen.
-      _counter++;
-    });
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    // This method is rerun every time setState is called, for instance as done
-    // by the _incrementCounter method above.
-    //
-    // The Flutter framework has been optimized to make rerunning build methods
-    // fast, so that you can just rebuild anything that needs updating rather
-    // than having to individually change instances of widgets.
-    return Scaffold(
-      appBar: AppBar(
-        // TRY THIS: Try changing the color here to a specific color (to
-        // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
-        // change color while the other colors stay the same.
-        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
-        // Here we take the value from the MyHomePage object that was created by
-        // the App.build method, and use it to set our appbar title.
-        title: Text(widget.title),
-      ),
-      body: Center(
-        // Center is a layout widget. It takes a single child and positions it
-        // in the middle of the parent.
-        child: Column(
-          // Column is also a layout widget. It takes a list of children and
-          // arranges them vertically. By default, it sizes itself to fit its
-          // children horizontally, and tries to be as tall as its parent.
-          //
-          // Column has various properties to control how it sizes itself and
-          // how it positions its children. Here we use mainAxisAlignment to
-          // center the children vertically; the main axis here is the vertical
-          // axis because Columns are vertical (the cross axis would be
-          // horizontal).
-          //
-          // TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
-          // action in the IDE, or press "p" in the console), to see the
-          // wireframe for each widget.
-          mainAxisAlignment: .center,
-          children: [
-            const Text('You have pushed the button this many times:'),
-            Text(
-              '$_counter',
-              style: Theme.of(context).textTheme.headlineMedium,
-            ),
-          ],
-        ),
-      ),
-      floatingActionButton: FloatingActionButton(
-        onPressed: _incrementCounter,
-        tooltip: 'Increment',
-        child: const Icon(Icons.add),
+  static ThemeData _theme(Brightness brightness) {
+    return ThemeData(
+      colorScheme: ColorScheme.fromSeed(
+        seedColor: const Color(0xFF4C6EF5),
+        brightness: brightness,
       ),
     );
   }

lib/pages/account_page.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+import '../widgets/empty_state.dart';
+
+/// 账户：登录、同步与偏好设置。
+class AccountPage extends StatelessWidget {
+  const AccountPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('账户')),
+      body: const EmptyState(
+        icon: Icons.person_outline,
+        title: '账户与同步',
+        message: '登录后可以在多台设备之间同步学习记录。',
+      ),
+    );
+  }
+}

lib/pages/manage_page.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+import '../widgets/empty_state.dart';
+
+/// 管理：牌组与卡片的增删改查。
+class ManagePage extends StatelessWidget {
+  const ManagePage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('管理')),
+      body: const EmptyState(
+        icon: Icons.library_books_outlined,
+        title: '管理卡片与牌组',
+        message: '在这里添加、编辑和整理你的学习材料。',
+      ),
+    );
+  }
+}

lib/pages/stats_page.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+import '../widgets/empty_state.dart';
+
+/// 统计：学习进度、记忆保持率与每日复习量。
+class StatsPage extends StatelessWidget {
+  const StatsPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('统计')),
+      body: const EmptyState(
+        icon: Icons.insights_outlined,
+        title: '复习数据统计',
+        message: '学习进度、记忆保持率和每日复习量会显示在这里。',
+      ),
+    );
+  }
+}

lib/pages/study_page.dart

@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+import '../widgets/empty_state.dart';
+
+/// 学习：今天到期的卡片与复习会话入口。
+class StudyPage extends StatelessWidget {
+  const StudyPage({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('学习')),
+      body: const EmptyState(
+        icon: Icons.school_outlined,
+        title: '开始今天的复习',
+        message: '待学习的卡片和到期需要复习的卡片会出现在这里。',
+      ),
+    );
+  }
+}

lib/widgets/empty_state.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+
+/// 各主页面共用的占位内容：一个图标、一句标题和可选说明。
+///
+/// 页面接上真实业务后可以直接替换掉。
+class EmptyState extends StatelessWidget {
+  const EmptyState({
+    required this.icon,
+    required this.title,
+    this.message,
+    super.key,
+  });
+
+  final IconData icon;
+  final String title;
+  final String? message;
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    final String? message = this.message;
+
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 32),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: <Widget>[
+            Icon(icon, size: 56, color: theme.colorScheme.primary),
+            const SizedBox(height: 16),
+            Text(
+              title,
+              style: theme.textTheme.titleMedium,
+              textAlign: TextAlign.center,
+            ),
+            if (message != null) ...<Widget>[
+              const SizedBox(height: 8),
+              Text(
+                message,
+                style: theme.textTheme.bodyMedium?.copyWith(
+                  color: theme.colorScheme.onSurfaceVariant,
+                ),
+                textAlign: TextAlign.center,
+              ),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}