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/links-menu.js

@@ -1,5 +1,5 @@
 // Pages menu in the dashboard chrome: one button that opens a popover
-// linking the standalone pages (flow, stats, API docs) and the swarm UI.
+// linking the standalone pages (stats, API docs) and the swarm UI.
 // Modelled on the swarm UI's `LinksMenu`. The swarm entry only exists
 // when `/api/state` carries `swarm_public_url`, so a hive without a
 // configured swarm domain shows no dead link.
@@ -10,7 +10,6 @@ import { el } from "@hive/shared/dom.js";
 /** Menu entries for a state snapshot. `external` opens in a new tab. */
 export function menuLinks(state) {
   const links = [
-    { icon: "📡", label: "flow", url: "/flow.html" },
     { icon: "📊", label: "stats", url: "/stats.html" },
     { icon: "🧬", label: "api docs", url: "/api/docs/" },
   ];