Keyboard shortcuts: fix warnings and add one for focusing chat input

[ellisonbg]

A few minor issues with keyboard shortcuts with Jupyter AI 3.0.0a0 pre-release.

Description

First, I see the following warnings in the browser console when using Jupyter AI 3.0.0a0 pre-release:

Second, when I have a chat document open and active, pressing enter doesn't focus the text box like I would expect.

Reproduce

1. Open chat document.
2. Make sure the text box is blurred, while the document is active.
3. Press enter to focus the text box.
4. See that it doesn't get focus (mouse click is required).

Expected behavior

• As a user, I want to use enter to focus the text box of a chat document.