relay offline mailbox

demonstrates store-and-forward: send to an address while it's offline, the relay buffers the message, then flushes it the moment that address registers - no polling, no accounts. back to docs

try it: 1) leave bob disconnected, 2) hit send on the left a few times, 3) click "connect bob now" and watch the backlog flush with the queued count, 4) bob's incoming messages auto-ack back to alice.