Persistent cache for Send access tokens, so repeat bw receive / bw send receive calls for
the same Send within a token’s validity window don’t re-prompt for a password or OTP.
bw receive <url>. Also used directly as the payload of SendCommands::Receive (bw send receive) — the two are the same command reachable under two names, sharing this single arg
struct (rather than a hand-synced copy) so the flag sets can’t drift apart. Both delegate to
receive::run_receive.