Provision a Stalwart group (team@example.org) with carol as a member before her
first login, and assert the composer's From selector offers the group address.
This confirms the group-membership scenario of #569 already works out of the
box: Stalwart returns the group's send-as identity on the member's own account,
so the app's normal single-account identity load surfaces it (identities.length
> 1 -> the From <select> renders with team@).
- stalwart: create the `team` Group in plan-accounts and add carol via
User.memberGroupIds in the entrypoint (id resolved after apply, like
DOMAIN_ID). carol, not alice/bob, so the sync specs stay unshared.
- helpers: GROUP config, openComposer/composerFromOptions, and JmapClient
accounts + sharedAccountNames (Identity/get needs the submission capability).
- composer: add data-testid="composer-from" to the From <select> and its
single-identity <span> fallback.
Ref: https://github.com/bulwarkmail/webmail/issues/569
