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

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。',
+      ),
+    );
+  }
+}