Find Stalwart's administration instead of asking for it

The dashboard's link to Stalwart's own administration needed
STALWART_ADMIN_URL, which an operator had to know to set. Everything it
holds can be read from the server:

- the origin is the host Stalwart advertises in its own session URLs, the
  one people reach it at even when ihasmail talks to it on a private
  address;
- the prefix is where its web interface application is installed. The
  production server's x:Application reads "Stalwart Web Interface",
  enabled, urlPrefix {"/admin", "/account"}, and /admin is also what
  Stalwart writes at first boot.

So, for an account that administers, the account info fetch now asks the
account's own server for its applications and links to origin + /admin/.
An installation whose web interface is disabled or moved gets no link; an
administrator who may not read applications gets Stalwart's default
/admin. It is cached with the rest of the account info.

STALWART_ADMIN_URL and a servers file entry's adminUrl still win, for an
administration that lives somewhere else. A routed domain without one now
takes what its own server said, never the default server's.

FEATURES.md

@@ -1153,11 +1153,15 @@ so rows come out even: six are three over three, and fall to two and then one
 as the space narrows. **Refresh** reads everything again; nothing is polled.
 
 Below the cards, a line 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` — or, for a domain routed to
-another server, that server's `adminUrl` in the servers file — and is plain text
-otherwise, since the address ihasmail reaches Stalwart on is often not one a
-browser can open.
+queue, logs and server settings are in Stalwart's own administration, and it
+links there. The address is found rather than configured: the public host
+Stalwart advertises in its own session — the one people reach it at, even when
+ihasmail talks to it on a private address — and the prefix its web interface is
+installed under, read from its `x:Application` objects (`/admin` unless it was
+moved). A server whose web interface is disabled or moved away gets no link, and
+an administrator who may not read applications gets Stalwart's default `/admin`.
+`STALWART_ADMIN_URL`, or a servers file entry's `adminUrl`, overrides it for an
+administration that lives somewhere else.
 
 ## Accounts
 
@@ -1715,7 +1719,7 @@ wizard, because either would be state.
 | --- | --- | --- |
 | `STALWART_URL` | — | Where Stalwart is; the JMAP session is discovered at `/.well-known/jmap` |
 | `SHOW_ENTERPRISE_NOTICES` | `0` | Say an Enterprise-only section (Tenants) is Enterprise-only even when the server is Enterprise. For a demo that reports Enterprise to show those sections; a real installation leaves it off |
-| `STALWART_ADMIN_URL` | — | Where a browser opens Stalwart's own administration, linked from the Administration dashboard. Separate from `STALWART_URL`, which is often an address only this server can reach; unset, the dashboard names Stalwart's administration without a link |
+| `STALWART_ADMIN_URL` | found | Where a browser opens Stalwart's own administration, linked from the Administration dashboard. Unset, it is found: the host Stalwart advertises and its web interface's prefix. Set it only when the administration lives somewhere else |
 | `APP_SECRET` | — | Key material for sealing sessions. **Required in production** — the server refuses to start without it |
 | `HOST` / `PORT` | `0.0.0.0` / `8080` | Listen address |
 | `BASE_PATH` | — (the domain root) | Subpath to serve from, e.g. `/mail`. Must be set for the **build** as well as the run — see below |