docs: add contributor and Colyseus guides

feat(client): parametrize WS/API via VITE_WS_URL and VITE_API_URL; add .env.example and README env section

client/.env.example

@@ -0,0 +1,10 @@
+# Client environment variables (copy to client/.env)
+
+# WebSocket endpoint for Colyseus server
+# e.g., ws://localhost:3000 (dev) or wss://api.my-domain.com (prod)
+VITE_WS_URL=ws://localhost:3000
+
+# REST API base (must include /api)
+# e.g., http://localhost:3000/api (dev) or https://api.my-domain.com/api (prod)
+VITE_API_URL=http://localhost:3000/api
+