Point from the dashboard to Stalwart's own administration

A line under the cards says where the rest is: detailed metrics, the
delivery queue, logs and server settings are in Stalwart's own
administration. It links there when the operator sets STALWART_ADMIN_URL,
and stays plain text otherwise, because STALWART_URL is how this server
reaches Stalwart and is often an address no browser can open.

Several servers: a servers file entry may now be an object,
{"url": ..., "adminUrl": ...}, and a session routed to that server gets its
adminUrl. A routed domain without one gets no link rather than the default
server's, for the same reason routing never falls back. The URL is sent
only to a session that may administer.

The shipped example file stopped the server at startup: its "_comment"
key was read as a domain and refused as not a URL, while the test that
checks the example skipped it. Keys starting with an underscore are notes
now -- no mail domain starts with one -- and the example is also loaded
through the real parser in a test, so the two cannot disagree again.

Two new strings, in all nine catalogues.

server/src/upstream.ts

@@ -54,6 +54,21 @@ export function upstreamFor(username: string): string {
   return config.stalwartServers[domain] ?? config.stalwartUrl;
 }
 
+/**
+ * Where the administrator signed in as `username` opens Stalwart's own
+ * administration, or null when the operator has not said.
+ *
+ * Follows the same routing as `upstreamFor`, and for the same reason never
+ * falls back: a domain routed to another server is not pointed at the default
+ * server's administration, where its accounts are not.
+ */
+export function adminUrlFor(username: string): string | null {
+  const at = username.lastIndexOf("@");
+  const domain = at < 0 ? "" : username.slice(at + 1).trim().toLowerCase().replace(/\.$/, "");
+  if (domain && domain in config.stalwartServers) return config.stalwartAdminUrls[domain] ?? null;
+  return config.stalwartAdminUrl || null;
+}
+
 export function wellKnownUrl(base: string = config.stalwartUrl): string {
   return `${base}/.well-known/jmap`;
 }