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/shared/src/chrome.css

@@ -1,6 +1,6 @@
 /* ─── page chrome: back-link header ────────────────────────────────
    The minimal sticky "frosted bar" header used by every standalone page
-   reached from the dashboard's pages menu — /flow, /stats. A back
+   reached from the dashboard's pages menu — /stats. A back
    link to the dashboard on the left, then a page title and, on /stats, a
    sub-tab nav. The operator dashboard uses its own richer
    sticky chrome (`.dashboard-chrome`); these classes cover the simple