feat: Implement Inbound Typing Indicators Subscription

[RokayaOthman]

The UI currently broadcasts outbound typing states via MessageInput.js, but it never listens for incoming typing alerts. No one can see when another teammate is typing.

• Proposed Implementation Steps:

1. In ChatPage.jsx, subscribe the initialized STOMP client to /topic/channel/{id}/typing whenever an activeChannel shifts.
2. Maintain a local typingUsers state array containing user IDs currently active.
3. Render a clean text notification (e.g., "Ahmed Aly is typing...") right above the MessageInput component using usersMap to resolve names.

• Acceptance Criteria:
• Subscribing to the typing broker functions seamlessly on channel change.
• UI displays an indicator when another user types and clears it automatically after a fallback timeout or clear event.