[FEATURE]: Ability to collapse/hide the session-chat-panel

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem:

I just tried opencode2's pair command to use it via the browser. Works well! I then connected to it from my phone via tailscale network. Works well!

However, when I look at the sidebar git changes diff view, I like being able to look at the side-by-side/Split diff. Of course, this would be pointless when the phone is in portrait orientation, which is probably why it defaults to Unified Diff (without any toggle to Split).

But when you change to landscape orientation, it changes to the full size/non-mobile UI where you can change to Split diff, but the chat session takes up half the screen.

Proposed Solution

There's already a button in the top right to show/hide the sidebar. It would be great if you can add a similar button on the left to show/hide the chat view. It would probably be generally useful, even on desktop.