Warn the user before navigating away, when the outbox has messages in it.

This is in lieu of saving the outbox. I tried that, and:

* If messages are dropped from the saved outbox before calling `api.postToChannel`, then messages "in flight" are lost when the page is reloaded unless the send succeeds after the client vanishes, as they are not re-sent when the page loads.
* If messages are dropped from the saved outbox after calling `api.postToChannel`, then messages "in flight" are duplicated when the page is reloaded and they get re-sent.

The CAP theorem is real and can hurt you.

The appropriate compensating mechanism would be a client-generated per-operation nonce, with server-side support for replaying responses by nonce if an operation already completed. That's a pretty big undertaking, and it's one we should probably do, but it's larger than I want to take on right now. Instead, we warn the user, and they can make their own decision.

Except we don't, sometimes.

When the client runs in a browser, this event handler prompts the user for confirmation before reloading, navigating away, closing the tab, or quitting. When run in a Safari app container, though, it only warns before reloading. Closing the window or quitting the app do not provoke a prompt. The warning is "best" effort. The failure mode is lost messages, which isn't particularly best.

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