feat: add student/class statistics page with Chart.js visualizations

app/templates/base.html

@@ -10,6 +10,8 @@
     <link href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.11.0/font/bootstrap-icons.css" rel="stylesheet">
     <link href="https://cdn.jsdelivr.net/npm/easymde/dist/easymde.min.css" rel="stylesheet">
     <link href="https://cdn.jsdelivr.net/npm/tabulator-tables@5/dist/css/tabulator.min.css" rel="stylesheet">
+    <!-- Chart.js -->
+    <script src="https://cdn.jsdelivr.net/npm/chart.js@4.4.0/dist/chart.umd.min.js"></script>
     {% block extra_css %}{% endblock %}
 
     <style>
@@ -86,6 +88,9 @@
                     <small id="currentUserDisplay" class="text-light"></small>
                 </div>
                 <nav class="nav flex-column">
+                    <a class="nav-link {% if active_nav == 'statistics' %}active{% endif %}" href="/statistics">
+                        <i class="bi bi-bar-chart"></i> 数据统计
+                    </a>
                     <a class="nav-link {% if active_nav == 'students' %}active{% endif %}" href="/students">
                         <i class="bi bi-people"></i> 学员管理
                     </a>