Shared mailboxes: a second kind of account lock

A shared mailbox (support@, legal@) belongs to no one person: nobody
signs in to it, and the people assigned open it beside their own mail
at an access level an administrator chose. An account lock already is
most of that: it keeps receiving mail, refuses every sign-in, and its
delegates reach it through real grants on every container (so IMAP,
DAV and JMAP honor them), never including Share. So a shared mailbox is
a lock of a second kind (multi-account spec, MA-S; John, 2026-10-05).

Lock gains kind: "lock" (the default, so stored locks read as before)
or "sharedMailbox", set on create and fixed after. A shared mailbox:

- needs no reason to make, change or end;
- holds up to 100 people, where a lock holds 10;
- runs its own Sieve replies and redirects, so an automatic
  acknowledgement goes out (a lock answers no one);
- records only what is sent as it (audit_send_as, which now covers it),
  not AL-9's access and per-change records, which would bury the log
  for a busy desk;
- sends only as itself (MA-S3): From and Reply-To must be its own
  addresses, so answers come back to the mailbox and not to whoever
  replied; anything else is forbiddenFrom.

The session marks it delegation: {locked: true, kind: "sharedMailbox"},
so a front end that knows no kind still treats it as a lock. The
console's layout gains Management › Directory › Shared Mailboxes
(CustomComponent/SharedMailboxes).

Tests: the account lock suite now goes on to a shared mailbox: made
without a reason with twelve people, sign-in refused, the session's
kind, its vacation reply delivered, an answer sent as it and recorded
as the agent with no per-change records, and a Reply-To naming the
agent refused; a lock unit test reads a stored lock without a kind.
account_lock_tests, jmap_tests, audit_log_tests and imap_tests pass
(RocksDB).

crates/jmap-proto/src/object/inbuxa_account_lock.rs

@@ -28,6 +28,8 @@ pub enum AccountLockProperty {
     /// The locked account (on create; afterwards the same as `id`).
     AccountId,
     Name,
+    /// MA-S: `lock` (the default) or `sharedMailbox`; set on create only.
+    Kind,
     Reason,
     LockedAt,
     LockedBy,
@@ -53,6 +55,7 @@ impl Property for AccountLockProperty {
             AccountLockProperty::Id => "id",
             AccountLockProperty::AccountId => "accountId",
             AccountLockProperty::Name => "name",
+            AccountLockProperty::Kind => "kind",
             AccountLockProperty::Reason => "reason",
             AccountLockProperty::LockedAt => "lockedAt",
             AccountLockProperty::LockedBy => "lockedBy",
@@ -68,6 +71,7 @@ impl AccountLockProperty {
             b"id" => AccountLockProperty::Id,
             b"accountId" => AccountLockProperty::AccountId,
             b"name" => AccountLockProperty::Name,
+            b"kind" => AccountLockProperty::Kind,
             b"reason" => AccountLockProperty::Reason,
             b"lockedAt" => AccountLockProperty::LockedAt,
             b"lockedBy" => AccountLockProperty::LockedBy,