fixed some bugs

static/teacher.html

@@ -26,11 +26,33 @@
         <input id="password" type="password" autocomplete="current-password" required>
         <button class="full" data-working type="submit">登录后台</button>
         <div class="notice" id="loginNotice"></div>
-        <p class="hint">默认账号 teacher，默认密码 teacher123。正式使用前请通过环境变量修改。</p>
+        <p class="hint">默认账号 teacher，默认密码 teacher!@#。首次使用后请及时修改密码。</p>
       </form>
     </div>
 
     <div id="mainArea" class="hidden">
+      <section class="panel">
+        <div class="section-head"><h2>账号安全</h2></div>
+        <form class="form-line" id="teacherPasswordForm">
+          <input id="teacherOldPassword" type="password" autocomplete="current-password" placeholder="原密码" required>
+          <input id="teacherNewPassword" type="password" autocomplete="new-password" minlength="8" placeholder="新密码（至少 8 位）" required>
+          <input id="teacherConfirmPassword" type="password" autocomplete="new-password" minlength="8" placeholder="确认新密码" required>
+          <button data-working type="submit">修改密码</button>
+        </form>
+        <div class="notice" id="teacherPasswordNotice"></div>
+      </section>
+
+      <section class="panel">
+        <div class="section-head"><h2>分组设置</h2></div>
+        <form class="form-line" id="groupLimitForm">
+          <input id="groupLimit" type="number" min="2" max="100" required>
+          <span>人</span>
+          <button data-working type="submit">保存上限</button>
+        </form>
+        <p class="hint">调整后立即对所有班级生效；已有小组不会被自动拆分。</p>
+        <div class="notice" id="groupLimitNotice"></div>
+      </section>
+
       <section class="panel">
         <div class="section-head">
           <h2>班级管理</h2>