fix: resolve EADDRINUSE port conflict — Node.js now listens on internal port 4000

Nginx listens publicly on 2087/2083 and reverse-proxies to 127.0.0.1:4000.
Previously both Nginx and Node.js were trying to bind port 2087 causing the
nixpanel service to crash-loop. HOST changed to 127.0.0.1 so the API is
not directly reachable from outside.

Bumps version to 0.3.1.

server/src/config.ts

@@ -16,8 +16,8 @@ export const config = {
 
   server: {
     host: optional('HOST', '0.0.0.0'),
-    nixserverPort: parseInt(optional('NIXSERVER_PORT', '2087'), 10),
-    nixclientPort: parseInt(optional('NIXCLIENT_PORT', '2083'), 10),
+    nixserverPort: parseInt(optional('NIXSERVER_PORT', '4000'), 10),
+    nixclientPort: parseInt(optional('NIXCLIENT_PORT', '4001'), 10),
   },
 
   db: {