Gateway restart notices: explicit status{restarting} signal, correct timing

The app previously showed 'Gateway restarting' on every connection loss.
Now the gateway broadcasts status{state=restarting} on its shutdown path
(before closing the sockets), and the app:

- posts 'Gateway restarting' immediately on that frame (not on the
  socket-drop transition, which lags by the ~20s WS ping timeout)
- posts 'Gateway online' on the next reconnect only when the restart
  notice was posted (latch) - a plain network drop shows neither, just
  the reconnect banner
- drops the 'Gateway is restarting...' banner (replaced by the chat notice)

Docs (04-wire-protocol, frames.schema.json) updated: restarting is no
longer reserved. Test for the disconnect broadcast added to the local
hermes-agent test mirror (git-ignored, not committed).

docs/04-wire-protocol.md

@@ -280,7 +280,13 @@ messages only.
 ### `status`
 
 Gateway health state. Broadcast to all connected clients at startup
-(`state: "online"`); `restarting` / `degraded` are reserved for future use.
+(`state: "online"`) and to late joiners on `hello.ack`. The gateway also
+broadcasts `state: "restarting"` on its shutdown path (restart/stop), right
+before closing the sockets — the app posts the "Gateway restarting" chat
+notice immediately on that frame (the socket can take up to the ~20 s ping
+timeout to actually drop, so the notice must not wait for the disconnect);
+a plain network drop shows just the reconnect banner. `degraded` is reserved
+for future use.
 
 ```json
 {"type":"status","payload":{"state":"online"}}