test(integration): composer From offers shared/group identities (#569)

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

components/email/email-composer.tsx

@@ -2143,6 +2143,7 @@ export function EmailComposer({
                 </div>
               ) : identities.length > 1 ? (
                 <select
+                  data-testid="composer-from"
                   value={selectedIdentityId || primaryIdentity?.id || ''}
                   onChange={(e) => setSelectedIdentityId(e.target.value)}
                   className="flex-1 bg-transparent text-sm text-foreground outline-none cursor-pointer hover:text-muted-foreground transition-colors min-w-0 truncate"
@@ -2174,7 +2175,7 @@ export function EmailComposer({
                       })}
                 </select>
               ) : (
-                <span className="text-sm text-foreground flex-1 truncate">
+                <span data-testid="composer-from" className="text-sm text-foreground flex-1 truncate">
                   {subAddressTag ? (
                     <span className="font-mono">
                       {generateSubAddress(primaryIdentity?.email || '', subAddressTag, subAddressDelimiter)}