feat(sweep): ability to display stale sweep plans

Could we keep this response visible instead of dropping it? cos the the backend’s first post-restart response is `stale:true`, `status:'failed`, with the restart error. Dropping it exposes the fresh-plan form, whose `force:true` request can overwrite the failed plan before the user sees or acknowledges it.

_Originally posted by @parrth20 in https://github.com/joinmarket-webui/jam/pull/1339#discussion_r3632511257_