Fix polling new chat message when switching sessions

[julien-nc]

closes #154
(This replaces it)

Add a new endpoint to check if a session is in a "thinking" state (meaning it currently has a scheduled/running task).
Use this endpoint to know if we should start polling when switching sessions.

Extra changes:

• Prevent scheduling multiple llm tasks for one session
• Fix using incorrect session ID in run/poll methods in the frontend
• Adjust title generation using the same logic as message generation. Title is written by the listener. Polling starts when switching to a session that has a running/scheduled title generation.