Harden MVP runtime and fix packaged desktop white screen

.env

@@ -4,3 +4,12 @@
 
 PORT_BACKEND=4000        # Express REST API  →  http://localhost:4000
 PORT_FRONTEND_DEV=5173   # Vite dev server   →  http://localhost:5173
+PORT_FRONTEND_DEV_FALLBACK=5174  # Optional local fallback if 5173 is already in use
+
+# Derived URLs for this project (kept explicit to avoid cross-project confusion)
+APP_URL=http://localhost:5173
+VITE_API_URL=http://localhost:4000/api
+API_BASE=http://localhost:4000/api
+
+# Browser origins allowed to call backend
+CORS_ORIGIN=http://localhost:5173,http://localhost:5174,http://localhost:4000,http://10.0.0.252:5173,http://10.0.0.252:5174,null