fix(dev): respect $PORT env var in vite.config.ts

Preview/dev tooling that assigns a free port via $PORT (because the
project's usual 5173 is taken) had no way to make Vite actually bind
there — Vite ignores PORT by default and falls back to its own
auto-increment, so the dev server would come up on a different port
than the one the tooling proxied to.

vite.config.ts

@@ -16,6 +16,9 @@ export default defineConfig({
     },
   },
   server: {
+    // Preview tooling assigns a free port via $PORT; fall back to Vite's
+    // own default for plain `npm run dev`.
+    port: process.env.PORT ? Number(process.env.PORT) : 5173,
     // Same-origin proxy to the local Stalwart container: avoids CORS entirely
     // (VITE_API_BASE_URL stays empty in .env.development.local).
     proxy: {