Offer administration only on a device marked as your own

A session signed in without "This is my own device" can no longer
administer. The server withholds the account's permissions from it and the
JMAP proxy refuses registry methods beyond the account's own, the same gate
ADMINISTRATION=0 uses. A borrowed or shared machine is where nobody should
be able to reset a password or remove a domain.

An administrator in such a session still sees Administration in the account
menu, greyed out, with the reason and the fix: sign in again with the box
ticked. The server tells that session only that the account administers.

The gate now reads the body only when it could name a registry method --
"x: in the text, or a \u escape that could spell one -- so ordinary mail
traffic from an untrusted session is forwarded untouched.

1 new string, translated in all nine catalogues, quoting each language's own
label for the tickbox; strings falling back to English stay at 16.

FEATURES.md

@@ -1166,6 +1166,20 @@ For a role that can read domains (`sysDomainQuery`, `sysDomainGet`):
 Switching DNS, DKIM or certificate management between automatic and manual,
 and choosing a DNS or ACME provider, stay in Stalwart's own interface for now.
 
+## Only on your own device
+
+Administration is available only to a session signed in with **"This is my own
+device"** ticked. A borrowed laptop or a shared machine is exactly where nobody
+should be able to reset a password or remove a domain, and that tickbox is the
+one question the sign-in page already asks about where it is being used.
+
+It is enforced the same way as the switch below: an untrusted session is sent
+no permissions, and the JMAP proxy refuses registry methods beyond the account's
+own. The menu still shows **Administration** to an administrator in that
+session, greyed out, with the reason and what to do about it — signing in again
+with the box ticked — rather than losing the entry without a word. All the
+server tells that session is that the account administers, never what it may do.
+
 ## An operator can turn it off
 
 `ADMINISTRATION=0` at launch removes it for everyone, and not only from the
@@ -1373,6 +1387,7 @@ costs something to get wrong is the one that assumes the machine is yours.
 | Idle sign-out | after 5 minutes | none |
 | Kept on the computer | nothing | settings cache, recent addresses, username |
 | Background notifications | refused | available |
+| Administration | unavailable | available, if the role allows it |
 
 Local storage is gated on that answer for **reads** as well as writes — a
 machine trusted once still has residue, and honouring it would let a previous