docs: add control ui dev gatewayUrl note

docs/web/control-ui.md

@@ -134,3 +134,29 @@ pnpm ui:dev # auto-installs UI deps on first run
 ```
 
 Then point the UI at your Gateway WS URL (e.g. `ws://127.0.0.1:18789`).
+
+## Debugging/testing: dev server + remote Gateway
+
+The Control UI is static files; the WebSocket target is configurable and can be
+different from the HTTP origin. This is handy when you want the Vite dev server
+locally but the Gateway runs elsewhere.
+
+1) Start the UI dev server: `pnpm ui:dev`
+2) Open a URL like:
+
+```text
+http://localhost:5173/?gatewayUrl=ws://<gateway-host>:18789
+```
+
+Optional one-time auth (if needed):
+
+```text
+http://localhost:5173/?gatewayUrl=wss://<gateway-host>:18789&token=<gateway-token>
+```
+
+Notes:
+- `gatewayUrl` is stored in localStorage after load and removed from the URL.
+- `token` is stored in localStorage; `password` is kept in memory only.
+- Use `wss://` when the Gateway is behind TLS (Tailscale Serve, HTTPS proxy, etc.).
+
+Remote access setup details: [Remote access](/gateway/remote).