dashboard: remove the flow page and the endpoints only it used

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.

frontend/packages/dashboard/src/dashboard.css

@@ -180,8 +180,6 @@ body.dashboard-shell {
   display: none;
 }
 
-/* FL0W is a separate page (`/flow.html`) — its full-viewport
-   styling lives in flow.css. */
 .banner {
   text-align: center;
   margin: 0 0 1em 0;