Support Cmd+Up and Cmd+Down to jump to top and bottom of thread

What version of the Codex App are you using (From "About Codex" dialog)?

_No response_

What subscription do you have?

_No response_

What platform is your computer?

macOS

What issue are you seeing?

In Codex, Cmd+Up and Cmd+Down do not appear to jump to the very top or very bottom of the current scrollable thread.

On macOS, this is expected keyboard behavior in many scrollable text/document views, and it would be especially useful in long Codex conversations where scrollbar dragging or precise scroll positioning can be awkward.

Possibly related: #17536

What steps can reproduce the bug?

1. Open a long Codex thread on macOS.
2. Press Cmd+Up.
3. Press Cmd+Down.
4. Observe that the view does not jump to the top/bottom of the thread.

What is the expected behavior?

• Cmd+Up scrolls/jumps to the very top of the current thread.
• Cmd+Down scrolls/jumps to the very bottom/latest message of the current thread.

Additional information

_No response_