Integrate chat support with live WebSocket streaming

[vikasbolla]

This PR integrates a real-time chat support system that allows users to contact admins directly from the power-access-cloud UI.

• User chat widget — users can open a chat panel, send messages, view history, and end conversations. The connection is maintained over WebSocket so messages are delivered instantly without polling.
• Admin chat panel — admins can see all active conversations with live unread indicators, open any conversation to read the full history, reply in real time, and end conversations on behalf of users.
• WebSocket infrastructure — an in-memory pub/sub hub routes messages between the user's session goroutine and the admin's session goroutine without any database polling. Admin replies reach the user's browser in microseconds.
• Persistent history — all messages are stored in MongoDB. When a user reconnects or navigates back to the chat, their full conversation history is restored. Ended conversations are recorded with a sentinel so the state survives server restarts.
• Async DB writes — admin replies are written to the database via a background worker queue so the admin UI never blocks waiting for a DB round trip.