docs(dev): explain the combined Vite and FastAPI launcher

docs/client/usage.md

@@ -9,23 +9,35 @@ reasoning, caller context, ApiPilot tools and load-balancer requests.
 From the repository root:
 
 ```bash
-SPEECH_TO_SPEECH_URL=ws://localhost:8765/v1/realtime ./start_ui.sh
+S2S_PROXY_TARGET=ws://localhost:8765/v1/realtime ./start_ui.sh
 ```
 
 The launcher loads the root `.env`, creates `.venv` if needed, installs the
-client requirements when the dependency check fails, and runs uvicorn on port
-8420. `UI_PORT` overrides the UI port. Because `.env` is sourced by the script,
-values in that file can replace variables supplied on the command line.
+client requirements when the dependency check fails, and installs the locked
+frontend dependencies when needed. Vite serves the React frontend on port
+8420 and proxies the backend routes to Uvicorn on `127.0.0.1:8421`.
+`UI_PORT` overrides the public port; `S2S_BACKEND_PORT` overrides the internal
+port. Both servers reload source edits, and stopping the launcher stops both.
+The speech pipeline remains a separate service. Because `.env` is sourced by
+the script, values in that file can replace variables supplied on the command line.
 
-To install and start the UI manually:
+Use Node.js 22.18+ on the 22.x line or Node.js 24+, npm, Python 3 with venv,
+Bash 4.4+, `setsid`, and `curl`. Remote staging access also needs the exact
+hostname in `S2S_DEV_ALLOWED_HOSTS`, HTTPS, and WebSocket forwarding for speech
+and Vite HMR. See [frontend development setup](../../frontend-s2s/README.md).
+
+To install and start the backend manually:
 
 ```bash
 python3 -m venv .venv
 .venv/bin/python -m pip install -r s2s-client/requirements.txt
-SPEECH_TO_SPEECH_URL=ws://localhost:8765/v1/realtime \
-  .venv/bin/python -m uvicorn --app-dir s2s-client server:app --port 8420
+S2S_PROXY_TARGET=ws://localhost:8765/v1/realtime \
+  .venv/bin/python -m uvicorn --app-dir s2s-client server:app \
+  --host 127.0.0.1 --port 8421 --reload --reload-dir s2s-client
 ```
 
+Then, in a second terminal, run `cd frontend-s2s`, `npm ci`, and `npm run dev`.
+
 Open <http://localhost:8420/>, tap the orb/microphone control, grant microphone
 access, and speak. Microphone access requires HTTPS or a local browser origin;
 use HTTPS when accessing the UI from another machine.
@@ -72,6 +84,9 @@ server; it is not a universally available local service.
 
 ## Docker
 
+This existing image serves the retained static client. It does not run the
+React/Vite development launcher.
+
 Use the project root as the build context:
 
 ```bash