dev: make the client's dev-server proxy target configurable via WUTZ_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.

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',
+      '/api': serverTarget,
-      '/admin': 'http://localhost:3000',
+      '/admin': serverTarget,
-      '/healthz': 'http://localhost:3000',
+      '/healthz': serverTarget,
     },
   },
 });