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

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。',
+      ),
+    );
+  }
+}