This PR adds a configurable max-width to the agent panel. This will be
particularly useful when opting into an agentic-first layout where the
thread will be at the center of the UI (with the panel most likely
full-screen'ed, which is why I'm also adding here the button to make it
full screen in the toolbar). The default max-width is 850, which is a
bit bigger than the one generally considered as a standard (~66
characters wide, which usually sums up to 750 pixels).

Release Notes:

- Agent: Added a max-width to the thread view for better readability,
particularly when the panel is zoomed in.