The loopback listener served one connection at a time with no read
timeout, so a connection that never sent a request (a browser preconnect,
any local process) held up the real form_post until the 5-minute wait
ran out. Each connection now runs on its own task and the first final
answer wins. The callback body is capped at 16 KiB; over the cap it reads
as empty and can't complete the login.
A request the gateway answers 401 now triggers one refresh (under the
cache lock, skipped if another process already replaced the refused
token) and one retry; a second 401 says to run `swarmctl login`.
The login cache directory is a `Swarm` field so tests can point it at a
scratch directory. New tests: an idle connection does not block the real
callback; an oversized body is refused without ending the wait; two
concurrent refreshers refresh once and share the token; a refused token
is refreshed once and retried; a second 401 says to log in without a
further retry; with no `--swarm`, the verbs use the controller socket.
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