cmd/tsconnect: use the parent window for beforeunload event listener

The SSH session may be rendered in a different window that the one that
is executing the script.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f