About: name the webmail node and the mail server node

Settings > About shows which webmail node answered (NODE_NAME, else the
container hostname) and which inbuxa node it talks to: the address the
server's name resolves to from the webmail, named by its PTR record. The
server only tells administrators its node name, so the webmail works it
out itself. Fetched from /api/about/nodes on every visit, cached for a
minute server-side, for troubleshooting a cluster.

Fork-only: upstream ihasmail runs one webmail against one server.
4 new strings, translated in all 9 catalogues.

web/src/locales/zh-Hans.ts

@@ -1720,6 +1720,11 @@ export const catalog: Catalog = {
     "To confirm, type {phrase}": "请输入 {phrase} 以确认",
     "Turn off legacy protocols": "关闭传统邮件协议",
     "Legacy mail protocols are off for your organization. Only {app} and JMAP apps can sign in.": "您的组织已关闭传统邮件协议。只有 {app} 和 JMAP 应用可以登录。",
+    // ── About: webmail and mail server nodes (inbuxa) ──────────────
+    "Webmail node": "网页邮箱节点",
+    "Mail server node": "邮件服务器节点",
+    "{host} does not resolve": "无法解析 {host}",
+    "unavailable": "不可用",
     // ── Spam filter: the language model's opinion (inbuxa) ──────────
     "Language model's opinion": "语言模型的判断",
     "One of several signals the spam filter weighed": "垃圾邮件过滤考虑的多个信号之一",