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

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。',
+      ),
+    );
+  }
+}