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.

lib/api/index.ts

@@ -1,6 +1,5 @@
 export { login, logout, register, resetPassword } from "./auth"
 export { sendForgotPasswordCode } from "./auth-extra"
-export { getChatSessionById, listChatMessages, listChatSessions } from "./chat"
 export { requestWithAuth } from "./client"
 export { addComment, listCommentsByPost, toggleCommentLike } from "./comments"
 export { getDisputeByOrderId, listDisputes } from "./disputes"