Let an operator turn administration off

ADMINISTRATION=0 at launch removes in-app administration for everyone. The
account's permissions are no longer sent to the browser, so the menu never
appears, and the JMAP proxy refuses Stalwart registry methods other than the
account's own (settings, password, app passwords, API keys, public keys,
masked addresses). Hiding the menu alone would have left an administrator's
browser console able to make every call the menu made.

With administration on, the request body streams through untouched as before;
only an installation that turns it off reads and checks the body, forwarding
the parsed form so the server receives exactly what was inspected.

FEATURES.md

@@ -1140,6 +1140,16 @@ in as it. ihasmail shows any account that outranks the viewer read-only, and
 counts a role it cannot read as outranking rather than not. Nobody can change
 their own role or delete the account they are signed in with.
 
+## An operator can turn it off
+
+`ADMINISTRATION=0` at launch removes it for everyone, and not only from the
+menu. The permissions are no longer sent to the browser, and the JMAP proxy
+refuses Stalwart registry methods except the ones about the signed-in account
+itself — its password, app passwords, API keys, public keys, masked addresses
+and account settings. Without that, hiding the menu would leave an
+administrator's browser console able to make every call the menu made.
+Stalwart's own interface is unaffected; this decides what ihasmail offers.
+
 ## Stateless, as everything else
 
 Nothing new is stored anywhere. There is no admin route on ihasmail's server,
@@ -1521,6 +1531,7 @@ wizard, because either would be state.
 | `UPSTREAM_TIMEOUT` | `30000` | Milliseconds |
 | `MAX_UPLOAD_BYTES` | `52428800` | 50 MB |
 | `IMAGE_PROXY` | `1` | Privacy proxy for remote images |
+| `ADMINISTRATION` | `1` | Offer in-app administration to accounts whose Stalwart role allows it; `0` turns it off, in the proxy as well as the menu |
 | `LOGIN_RATE_LIMIT` | `10` | Attempts per window |
 | `COOKIE_NAME` | `ihm_session` | |
 | `APP_NAME` | `ihasmail` | Branding |