This PR adds the token count to the active thread view. It doesn't
behaves quite like Assistant 1 where it updates as you type, though; it
updates after you submit the message.

<img
src="https://github.com/user-attachments/assets/82d2a180-554a-43ee-b776-3743359b609b"
width="700" />

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Release Notes:

- agent: Add token count in the thread view

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Co-authored-by: Agus Zubiaga <hi@aguz.me>