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.

docs/web-ui/README.md

@@ -12,7 +12,7 @@ what you actually do here.
 
 Everything starts at the **dashboard**, served at `/`. A pages menu
 (the `☰` button at the left of the tab strip) links to the other
-surfaces — Flow, Stats, the API docs and the swarm UI — and each of
+surfaces — Stats, the API docs and the swarm UI — and each of
 those pages links back to the dashboard.
 
 The dashboard is where you'll spend most of your time. It's a single
@@ -26,9 +26,8 @@ page with five tabs:
 - **K3PT ST4T3** — kept state from agents with no container
   (tombstones), with a purge button per entry.
 
-Two things live on their own page instead, reached from the pages
-menu: **Flow** (`/flow.html`, the raw live message stream across the
-whole swarm) and **Stats** (`/stats.html`, swarm-wide usage stats).
+Usage stats live on their own page, reached from the pages menu:
+**Stats** (`/stats.html`, swarm-wide usage stats).
 
 Each agent also has its own page — a full terminal view of that
 agent's session, reachable by selecting its name anywhere in the