fix: make REACT_APP_BACKEND_URL configurable for remote deployments (fixes #352)

When AgenticSeek is deployed on a remote server and accessed from a
different machine, the frontend JavaScript runs in the user's browser
with localhost:7777 which resolves to the user's local machine, not
the server - causing the 'System offline' error even though the backend
is running.

Change docker-compose.yml to read REACT_APP_BACKEND_URL from .env
(defaulting to http://localhost:7777 for local use) and document the
variable in .env.example so users can point the frontend at their
server's public IP when deploying remotely.

docker-compose.yml

@@ -64,7 +64,7 @@ services:
     environment:
       - NODE_ENV=development
       - CHOKIDAR_USEPOLLING=true
-      - REACT_APP_BACKEND_URL=http://localhost:7777
+      - REACT_APP_BACKEND_URL=${REACT_APP_BACKEND_URL:-http://localhost:7777}
     networks:
       - agentic-seek-net