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/ja.ts

@@ -1721,6 +1721,11 @@ export const catalog: Catalog = {
     "To confirm, type {phrase}": "確認のため {phrase} と入力してください",
     "Turn off legacy protocols": "従来のメールプロトコルをオフにする",
     "Legacy mail protocols are off for your organization. Only {app} and JMAP apps can sign in.": "組織では従来のメールプロトコルがオフになっています。サインインできるのは {app} と JMAP アプリのみです。",
+    // ── About: webmail and mail server nodes (inbuxa) ──────────────
+    "Webmail node": "Webメールのノード",
+    "Mail server node": "メールサーバーのノード",
+    "{host} does not resolve": "{host} の名前解決ができません",
+    "unavailable": "利用できません",
     // ── Spam filter: the language model's opinion (inbuxa) ──────────
     "Language model's opinion": "言語モデルの見解",
     "One of several signals the spam filter weighed": "迷惑メールフィルターが考慮した複数の判断材料のひとつ",