I was testing out the changes in
#52730 and realized that the
agent panel, when full screen, would be auto-dismissed if I toggled the
sidebar off. Turns out this happens because we were "hard-coding" the
focus back to the center pane, which was automatically dismissing zoomed
items. So, in this PR, I essentially am copying the ModalLayer approach
of storing whatever was focused before so we can return focus back to it
if possible.

Release Notes:

- N/A

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.

…ies#53283)

I was testing out the changes in
zed-industries#52730 and realized that the
agent panel, when full screen, would be auto-dismissed if I toggled the
sidebar off. Turns out this happens because we were "hard-coding" the
focus back to the center pane, which was automatically dismissing zoomed
items. So, in this PR, I essentially am copying the ModalLayer approach
of storing whatever was focused before so we can return focus back to it
if possible.

Release Notes:

- N/A

Quick follow up to #52730 with
some adjustments. Ended up cleaning some unused UI elements already
(i.e., the "Recent" history items in the agent panel's empty state).

Release Notes:

- N/A