feat(chat): implement WebSocket chat client with useChatSocket hook

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.

next.config.ts

@@ -14,6 +14,10 @@ const nextConfig: NextConfig = {
         source: "/api/:path*",
         destination: `${backendUrl}/api/:path*`,
       },
+      {
+        source: "/healthz",
+        destination: `${backendUrl}/healthz`,
+      },
     ]
   },
 }