feat: 后台管理添加用户管理功能 + 前端用户注册登录调用backend API

www/admin.html

@@ -435,6 +435,10 @@
           <span class="sidebar-icon">📊</span>
           <span>统计信息</span>
         </div>
+        <div class="sidebar-item" data-page="users">
+          <span class="sidebar-icon">👥</span>
+          <span>用户管理</span>
+        </div>
         <div class="sidebar-item" data-page="llm">
           <span class="sidebar-icon">🧠</span>
           <span>大模型配置</span>