refactor: unify toolbar style with card wrapper in problems and classes pages

app/templates/classes.html

@@ -3,21 +3,25 @@
 {% block title %}班级管理 - 钢琴练习方案系统{% endblock %}
 
 {% block content %}
-<div class="d-flex justify-content-between align-items-center mb-4">
-    <div class="d-flex align-items-center gap-3">
-        <h4><i class="bi bi-collection"></i> 班级管理</h4>
-        <select class="form-select form-select-sm" style="width:auto;" id="activeFilter" onchange="loadClasses()">
-            <option value="">全部班级</option>
-            <option value="true" selected>进行中</option>
-            <option value="false">已结束</option>
-        </select>
-        <button class="btn btn-primary btn-sm active" id="mineFilterBtn" onclick="toggleMineFilter()">
-            <i class="bi bi-person"></i> 我的
-        </button>
+<div class="card mb-3">
+    <div class="card-body">
+        <div class="d-flex justify-content-between align-items-center">
+            <div class="d-flex align-items-center gap-3">
+                <h5 class="mb-0"><i class="bi bi-collection"></i> 班级管理</h5>
+                <select class="form-select form-select-sm" style="width:auto;" id="activeFilter" onchange="loadClasses()">
+                    <option value="">全部班级</option>
+                    <option value="true" selected>进行中</option>
+                    <option value="false">已结束</option>
+                </select>
+                <button class="btn btn-primary btn-sm" id="mineFilterBtn" onclick="toggleMineFilter()">
+                    <i class="bi bi-person"></i> 我的
+                </button>
+            </div>
+            <button class="btn btn-primary btn-sm" id="addClassBtn" style="display:none;">
+                <i class="bi bi-plus-circle"></i> 新增班级
+            </button>
+        </div>
     </div>
-    <button class="btn btn-primary" id="addClassBtn" style="display:none;">
-        <i class="bi bi-plus-circle"></i> 新增班级
-    </button>
 </div>
 
 <div class="card">