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.

server/src/config.ts

@@ -295,6 +295,13 @@ export const config = {
   upstreamTimeout: int("UPSTREAM_TIMEOUT", 30_000),
   maxUploadBytes: int("MAX_UPLOAD_BYTES", 50 * 1024 * 1024),
   imageProxy: bool("IMAGE_PROXY", true),
+  /*
+   * Whether ihasmail offers administration to accounts whose Stalwart role
+   * allows it. Off means off: no menu, no permissions sent to the browser, and
+   * the JMAP proxy refuses registry methods beyond the account's own -- see
+   * adminGate.ts. Stalwart's own interface is unaffected either way.
+   */
+  administration: bool("ADMINISTRATION", true),
   cookieName: env("COOKIE_NAME", "ihm_session"),
   staticDir: process.env.STATIC_DIR ?? fileURLToPath(new URL("../../web/dist", import.meta.url)),
   loginRateLimit: int("LOGIN_RATE_LIMIT", 10),