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

vite.config.ts hardcoded the /api, /admin, /healthz proxy targets to
http://localhost:3000 — the server's default port — so there was no
way to run dev:client against a dev:server started on a different
port (PORT=<n> pnpm dev:server) without editing the config file.

Read the port from WUTZ_SERVER_PORT (same default of 3000 as the
server's own PORT env var) and build the proxy target once.

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

client/vite.config.ts

@@ -3,6 +3,13 @@ import preact from '@preact/preset-vite';
 import legacy from '@vitejs/plugin-legacy';
 import { resolve } from 'node:path';
 
+// The dev-server proxy target — same default (3000) as the server's own
+// `process.env.PORT ?? 3000` in server/src/index.ts. Override with
+// `WUTZ_SERVER_PORT` when running `dev:server` on a non-default port, e.g.:
+//   PORT=4000 pnpm dev:server
+//   WUTZ_SERVER_PORT=4000 pnpm dev:client
+const serverTarget = `http://localhost:${process.env.WUTZ_SERVER_PORT ?? 3000}`;
+
 export default defineConfig({
   plugins: [
     preact(),
@@ -24,9 +31,9 @@ export default defineConfig({
   server: {
     port: 5173,
     proxy: {
-      '/api': 'http://localhost:3000',
-      '/admin': 'http://localhost:3000',
-      '/healthz': 'http://localhost:3000',
+      '/api': serverTarget,
+      '/admin': serverTarget,
+      '/healthz': serverTarget,
     },
   },
 });