Add Groups to Administration

A group is a shared address and mailbox and the people who share it. To
Stalwart it is an x:Account of type Group, behind the same sysAccount*
permissions as a person, so it sits under Directory beside Accounts:
search, a page of fifty with each group's member count, and a panel to
create, edit and delete one.

Membership lives on the member, not the group. Members are the users whose
memberGroupIds name it, and adding or removing one is a single
memberGroupIds/<group> pointer on that user's account -- true or null --
which leaves their other groups alone. Changes apply straight away rather
than riding on Save, so the list is always what the server has. Nobody can
add or remove themselves, the same line the account panel draws at one's
own role.

A group's role is Default or Custom, not a person's User or Admin, and it
is what the group may do: in 0.16 a user's permissions come from their own
roles only, and a group gives its members what is shared with it. Only
roles the viewer could grant are offered.

Delete takes the members out first and then deletes the group, the order
a domain's keys go before the domain, because the registry keeps anything
another object names. A role that cannot change the members' accounts is
not offered a delete it could only half finish.

The mock's groups had a person's roles, accepted a memberGroupIds filter
without applying it, and answered a linked delete with the wrong shape;
all three follow the source now, and it refuses nested groups and
memberships of things that are not groups.

Nothing about groups has been run against a live server yet: production
has none, and every operation is a write. KNOWN-ISSUES says what was read
from source.

Thirty-five new strings, two of them plurals, in all nine catalogues.

README.md

@@ -83,7 +83,7 @@ More, including the mobile layout, on [ihasmail.org](https://ihasmail.org/#scree
 - **Nine new interface languages** — German, Spanish, French, Dutch, Portuguese (Brazil), Russian, Ukrainian, Simplified Chinese and Japanese, alongside English and separate from the date-and-time locale. Every one is marked **Beta**: they were made by AI and no native speaker has read them yet, which Settings says plainly, with a link for reporting anything wrong
 - **Twelve themes** — Classic and ihasmail's own, plus Catppuccin, Dracula, Gruvbox, Rosé Pine, Tokyo Night, Solarized, Ayu, Kanagawa, Everforest and Primer, each with the light and dark half its own project publishes. Palette and light-or-dark are separate choices, and the accent colour still sits on top of any of them. Only published colour values are used, taken from each project's own repository; the shades between them are derived and every text colour is measured against the surface it sits on, so a palette that would not meet the contrast this app claims is not written at all — see [Themes](FEATURES.md#themes)
 - **On a phone** — swipe a message to archive or delete it (either direction, your choice), hold one to select it, hold a folder for its menu, pull the list to refresh, swipe back from a conversation
-- **Administration** — for an account whose Stalwart role manages accounts or domains, from the account menu: a dashboard of users, domains, queued mail, memory and the last day's received and sent, scoped to a tenant administrator's own tenancy; create, edit and delete accounts and set their passwords; add domains, copy their DNS records one at a time or as a zone file, see their DKIM keys, and remove them once nothing uses them. Each control is there only when the role allows it, and Stalwart decides every call. Only for a session signed in with *This is my own device* ticked, and `ADMINISTRATION=0` turns it off for everyone — see [Administration](FEATURES.md#administration)
+- **Administration** — for an account whose Stalwart role manages accounts or domains, from the account menu: a dashboard of users, domains, queued mail, memory and the last day's received and sent, scoped to a tenant administrator's own tenancy; create, edit and delete accounts and set their passwords; create groups and add or remove their members; add domains, copy their DNS records one at a time or as a zone file, see their DKIM keys, and remove them once nothing uses them. Each control is there only when the role allows it, and Stalwart decides every call. Only for a session signed in with *This is my own device* ticked, and `ADMINISTRATION=0` turns it off for everyone — see [Administration](FEATURES.md#administration)
 - **Platform** — installable PWA, Web Push with ihasmail closed, `mailto:` handler, no credentials in the browser, strict CSP, SSRF-safe image proxy
 
 The long version is on [ihasmail.org](https://ihasmail.org/#features); how to