fix(build): disable webpack build workers to reduce memory

Next.js spawns per-worker webpack processes — each has its own heap.
On low-RAM servers total usage exceeds available memory → SIGKILL (silent).
webpackBuildWorker: false runs compilation in main process only.
Also lower JS heap cap to 512MB to leave room for OS + pnpm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

next.config.ts

@@ -26,6 +26,9 @@ const withPWA = withPWAInit({
 const nextConfig: NextConfig = {
   output: "standalone",
   turbopack: {},
+  experimental: {
+    webpackBuildWorker: false,
+  },
 };
 
 export default withPWA(nextConfig);