readme: document WUTZ_SERVER_PORT for running dev:client against a non-default dev:server port

README.md

@@ -91,6 +91,14 @@ pnpm dev:client                            # http://localhost:5173 (proxies /api
 Open `http://localhost:5173/` for the tablet UI and
 `http://localhost:5173/admin.html` for the backoffice.
 
+Running `dev:server` on a non-default `PORT`? Point `dev:client`'s proxy at it with
+`WUTZ_SERVER_PORT` (same value as `PORT`):
+
+```sh
+PORT=4000 ADMIN_PASSWORD=changeme pnpm dev:server
+WUTZ_SERVER_PORT=4000 pnpm dev:client
+```
+
 ## 📦 Production build
 
 ```sh
@@ -156,3 +164,6 @@ the service user owns the database directory automatically.
 - `ADMIN_PASSWORD` (**required** for backoffice login)
 - `WUTZ_TZ` (default `Europe/Berlin`) — timezone for stats display and grouping
 - `WUTZ_DAY_CUTOFF_HOUR` (default `5`) — sales before this local hour count toward the previous business day
+- `WUTZ_SERVER_PORT` (default `3000`, **client dev only** — not read by the server) — the port
+  `dev:client`'s Vite proxy targets; set it to match `dev:server`'s `PORT` when running the server
+  on something other than the default