What changed: the Sessions page lists sessions seen only through the
gateway (from the usage ledger, read-only, a native session of the same id
wins), and shows in gateway mode. A Sessions switch, off by default and
confirmed before it turns on, records each gateway exchange's text under
the config dir (scrubbed, at most 7 days and 256 MiB, cleaned hourly).
Settings sync and restore can't turn it on; POST /api/sessions/recording
turns it on or off and can clear what was kept.
Why: users of magpie as a gateway (gateway mode, LAN clients) had no way
to see their conversations; the owner chose this design on the PR.
Squashed from PR head 6a343ba1 onto origin/main. Conflicts: gui-shell.md
(kept main's placeMenu text, took the PR's Sessions in gateway mode) and
settings_keep_test.go (main's fields plus GatewayConversations).
Run: go build ./..., go vet on gateway, gui, sessions, settings, usage;
go test -race -timeout 900s on those five packages: all ok.