The flow page showed the raw broker message stream and had a compose box.
This removes the page and its bundle, its link in the pages menu, and the
styles only it used.
The history endpoint that backfilled it and the operator-send endpoint
behind the compose box had no other caller, so they go too, along with
the broker query behind the history endpoint. The stream endpoint stays:
the dashboard tabs still subscribe to it.
The docs lose the FL0W section and the references to the two endpoints.
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