feat: manage, account and help pages

- 管理页：单元集 / 计划 / 算法三个维度，卡片带元数据
- 账户页：资料、学习数据、竖直分组的四个功能入口
- 新增设置、文档与帮助（底部可上滑助手）、反馈、关于四个页面
- 抽出 StatBlock 与 GroupedListSection 共用组件
- 关闭 Android 预测式返回，转场改为 M3 水平滑入

lib/pages/account_page.dart

@@ -1,20 +1,166 @@
 import 'package:flutter/material.dart';
 
-import '../widgets/empty_state.dart';
+import '../models/account_profile.dart';
+import '../widgets/grouped_list_section.dart';
+import '../widgets/stat_block.dart';
+import 'about_page.dart';
+import 'docs_page.dart';
+import 'feedback_page.dart';
+import 'settings_page.dart';
 
-/// 账户：登录、同步与偏好设置。
+/// 账户页：资料、学习数据、功能入口。
 class AccountPage extends StatelessWidget {
   const AccountPage({super.key});
 
+  static const AccountProfile _profile = AccountProfile.sample;
+
   @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。',
+      body: ListView(
+        padding: const EdgeInsets.fromLTRB(16, 8, 16, 24),
+        children: const <Widget>[
+          _ProfileCard(profile: _profile),
+          SizedBox(height: 12),
+          _StatsCard(profile: _profile),
+          SizedBox(height: 22),
+          _ActionList(),
+        ],
       ),
     );
   }
 }
+
+class _ProfileCard extends StatelessWidget {
+  const _ProfileCard({required this.profile});
+
+  final AccountProfile profile;
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    final ColorScheme colors = theme.colorScheme;
+
+    return Container(
+      padding: const EdgeInsets.all(16),
+      decoration: BoxDecoration(
+        color: colors.surfaceContainerLow,
+        borderRadius: BorderRadius.circular(20),
+      ),
+      child: Row(
+        children: <Widget>[
+          CircleAvatar(
+            radius: 30,
+            backgroundColor: colors.primaryContainer,
+            child: Text(
+              profile.displayName.characters.first,
+              style: theme.textTheme.titleLarge?.copyWith(
+                color: colors.onPrimaryContainer,
+                fontWeight: FontWeight.w700,
+              ),
+            ),
+          ),
+          const SizedBox(width: 16),
+          Expanded(
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.start,
+              children: <Widget>[
+                Text(
+                  profile.displayName,
+                  style: theme.textTheme.titleMedium,
+                  maxLines: 1,
+                  overflow: TextOverflow.ellipsis,
+                ),
+                const SizedBox(height: 4),
+                Text(
+                  'ID ${profile.userId} · ${profile.stage}',
+                  style: theme.textTheme.bodySmall?.copyWith(
+                    color: colors.onSurfaceVariant,
+                  ),
+                ),
+              ],
+            ),
+          ),
+          Icon(
+            Icons.chevron_right_rounded,
+            color: colors.onSurfaceVariant,
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _StatsCard extends StatelessWidget {
+  const _StatsCard({required this.profile});
+
+  final AccountProfile profile;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      padding: const EdgeInsets.all(18),
+      decoration: BoxDecoration(
+        color: Theme.of(context).colorScheme.surfaceContainerLow,
+        borderRadius: BorderRadius.circular(20),
+      ),
+      child: Row(
+        children: <Widget>[
+          Expanded(
+            child: StatBlock(
+              label: '累计学习',
+              value: '${profile.totalHours}',
+              unit: '小时',
+            ),
+          ),
+          Expanded(
+            child: StatBlock(
+              label: '坚持',
+              value: '${profile.streakDays}',
+              unit: '天',
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+/// 竖直排列的长按钮列表，样式参考原生 Android 设置页。
+class _ActionList extends StatelessWidget {
+  const _ActionList();
+
+  static const List<(IconData, String)> _items = <(IconData, String)>[
+    (Icons.settings_outlined, '设置'),
+    (Icons.menu_book_outlined, '文档与帮助'),
+    (Icons.feedback_outlined, '反馈问题'),
+    (Icons.info_outline_rounded, '关于 KiriMemo'),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return GroupedListSection(
+      tiles: <Widget>[
+        for (final (IconData icon, String label) in _items)
+          GroupedListTile(
+            icon: icon,
+            title: label,
+            onTap: () => _open(context, label),
+          ),
+      ],
+    );
+  }
+
+  void _open(BuildContext context, String label) {
+    final Widget page = switch (label) {
+      '设置' => const SettingsPage(),
+      '文档与帮助' => const DocsPage(),
+      '反馈问题' => const FeedbackPage(),
+      _ => const AboutPage(),
+    };
+    Navigator.of(context).push(
+      MaterialPageRoute<void>(builder: (BuildContext _) => page),
+    );
+  }
+}