fix(dialog): AI 助手消息推送补全发送者身份

AI 助手为虚拟用户（userid=-1）无会员记录，userid2nickname 返回空串，
导致群聊推送拼成 ": 内容"。友盟 App 推送与前端桌面/移动通知改为取
msg.nickname 或默认"AI 助手"作为发送者名，显示为 "名称: 内容"。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Tasks/WebSocketDialogMsgTask.php

@@ -189,7 +189,12 @@ class WebSocketDialogMsgTask extends AbstractTask
         if ($umengUserid) {
             $setting = Base::setting('appPushSetting');
             if ($setting['push'] === 'open') {
-                $umengTitle = User::userid2nickname($msg->userid);
+                if ($msg->userid == -1) {
+                    // AI 助手虚拟用户没有会员记录，取自定义昵称或默认名称
+                    $umengTitle = ($msg->msg['nickname'] ?? '') ?: Doo::translate('AI 助手');
+                } else {
+                    $umengTitle = User::userid2nickname($msg->userid);
+                }
                 $umengBody = WebSocketDialogMsg::previewMsg($msg);
                 if ($dialog->type == 'group') {
                     $umengBody = $umengTitle . ': ' . $umengBody;