Replace the stubbed chat API with a real WebSocket hook that
connects to /ws/chat and supports DM creation, messaging,
session join/leave, and history retrieval. Keep REST stub
functions for backward compatibility with existing pages
that require listChatSessions/getChatSessionById imports.
Default backend URL changed from port 8080 (kubectl port-forward)
to 18080 (docker compose Envoy Gateway). README updated to reflect
the deploy/dev workflow.
zetaloop