Add Mailing lists to Administration

A mailing list is an address that passes mail on to everyone on it. To
Stalwart it is its own object, x:MailingList, behind sysMailingList*, so
it gets its own section under Directory after Groups: search, fifty to a
page with each list's recipient count, and a panel to create, edit and
delete one.

Recipients are a property of the list, so unlike a group's members they
save with the rest of the panel. What Save sends for them is only what was
added and removed, one recipients/<address> pointer each -- the patch the
live server accepted -- so a recipient added elsewhere while the panel was
open is not taken out. They can be pasted several at a time, from a
spreadsheet column, a comma-separated line or Name <address>; anything with
an @ that is not an address stays in the box with a note. Past a dozen, a
filter narrows them.

That is all a list is in Stalwart -- no owners, moderation or posting
rules -- so that is all the panel offers.

The mock answers x:MailingList with two lists, the recipient set's live
shape, and the refusals a wrong address, a clash with an account and a
missing permission get.

Twenty-five new strings and one plural, in all nine catalogues.

web/src/styles/app.css

@@ -1771,6 +1771,8 @@ select optgroup { background-color: var(--bg-elev); color: var(--fg); }
 .admin-add-member { margin-top: 10px; }
 .admin-add-member .admin-search { max-width: none; display: block; }
 .admin-suggestions { margin-top: 6px; }
+.admin-recipients { max-height: 220px; overflow-y: auto; padding: 2px 0; }
+.admin-recipient-filter { display: block; max-width: none; margin-bottom: 8px; }
 .admin-kv { display: grid; grid-template-columns: auto 1fr; gap: 8px 16px; align-items: center; margin: 0; font-size: .92em; }
 .admin-kv dt { color: var(--fg-muted); }
 .admin-kv dd { margin: 0; }