fix(aibox/frontend): Batch M 波次3 前端功能修复（F1/F2/F3/F4/F6）

- F1: entry/frontend 清空按钮先 cloneNode 缓存 welcome-message 再清空，修 TypeError 中断
- F2: 三页面手动断开清除/作废重连定时器，杜绝连接复活（entry 新增 reconnectTimer 句柄）
- F3: 三页面连接前关闭 CONNECTING 中旧连接，防孤儿连接/双后端会话
- F4: 6 个 CDN CSS link 补 SRI integrity+crossorigin（sha384 实测计算）
- F6: frontend 重连文案与 3000*N 实际延迟对齐

三页面内联脚本 node --check 通过；测试 493 passed/4 failed（既有平台项）/1 skipped 零回归

aibox/frontend.html

@@ -4,8 +4,8 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>AI智能助手 - WebSocket直连</title>
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.0.0/css/all.min.css">
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.8.0/styles/github-dark.min.css">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.0.0/css/all.min.css" integrity="sha384-3B6NwesSXE7YJlcLI9RpRqGf2p/EgVH8BgoKTaUrmKNDkHPStTQ3EyoYjCGXaOTS" crossorigin="anonymous">
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.8.0/styles/github-dark.min.css" integrity="sha384-wH75j6z1lH97ZOpMOInqhgKzFkAInZPPSPlZpYKYTOqsaizPvhQZmAtLcPKXpLyH" crossorigin="anonymous">
     <!-- J13（审计 2026-08-19）：CDN 脚本补 SRI integrity，防供应链劫持 -->
     <script src="https://cdnjs.cloudflare.com/ajax/libs/marked/9.1.6/marked.min.js" integrity="sha384-odPBjvtXVM/5hOYIr3A1dB+flh0c3wAT3bSesIOqEGmyUA4JoKf/YTWy0XKOYAY7" crossorigin="anonymous"></script>
     <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.8.0/highlight.min.js" integrity="sha384-g4mRvs7AO0/Ol5LxcGyz4Doe21pVhGNnC3EQw5shw+z+aXDN86HqUdwXWO+Gz2zI" crossorigin="anonymous"></script>
@@ -1424,9 +1424,12 @@
                     
                     if (!manualDisconnect && reconnectAttempts < MAX_RECONNECT_ATTEMPTS) {
                         reconnectAttempts++;
-                        showSystemMessage(`⚠️ 连接断开，${reconnectAttempts}/${MAX_RECONNECT_ATTEMPTS} 秒后重试...`);
+                        // F6（审计 2026-08-20）：实际延迟为 3000*N ms，文案与此对齐
+                        showSystemMessage(`⚠️ 连接断开，${3 * reconnectAttempts} 秒后重试（${reconnectAttempts}/${MAX_RECONNECT_ATTEMPTS}）...`);
                         setTimeout(() => {
-                            if (!ws || ws.readyState !== WebSocket.OPEN) {
+                            // F2（审计 2026-08-20）：闭包到期时再判 manualDisconnect，
+                            // 防止手动断开后已排程定时器把连接"复活"
+                            if (!manualDisconnect && (!ws || ws.readyState !== WebSocket.OPEN)) {
                                 console.log('尝试重连...');
                                 ws = connectWebSocket();
                             }
@@ -1596,7 +1599,8 @@
             
             // 事件绑定
             document.getElementById('connect-btn').addEventListener('click', function() {
-                if (ws && ws.readyState === WebSocket.OPEN) {
+                // F3（审计 2026-08-20）：CONNECTING 中的旧连接也要关闭，否则重复点击产生孤儿连接/双后端会话
+                if (ws && ws.readyState !== WebSocket.CLOSED) {
                     ws.close();
                 }
                 reconnectAttempts = 0;
@@ -1639,11 +1643,14 @@
             
             document.getElementById('clear-chat-btn').addEventListener('click', function() {
                 const container = document.getElementById('messages-container');
+                // F1（审计 2026-08-20）：welcome-message 在 container 内，
+                // 必须先 cloneNode 缓存再清空，否则 querySelector 返回 null 抛 TypeError，
+                // 且后续 clearAllStreamingMessages/计数清零全部中断不执行
+                const welcomeEl = document.querySelector('.welcome-message');
+                const welcomeHtml = welcomeEl ? welcomeEl.cloneNode(true) : null;
                 container.innerHTML = '';
-                
-                const welcomeHtml = document.querySelector('.welcome-message').cloneNode(true);
-                container.appendChild(welcomeHtml);
-                
+                if (welcomeHtml) container.appendChild(welcomeHtml);
+
                 clearAllStreamingMessages();
                 messageCount = 0;
                 toolCallCount = 0;