No way to close Leo if "show sidebar button" toggle is disabled

[Brave-Matt]

Description

If you have the sidebar button disabled (Settings --> Appearance ), when you go to Ask Leo in the address bar and ask it something, there’s then no way to close the sidebar panel. Your options are to close the tab entirely or to use the keyboard shortcut, which is not intuitive.

We should add x or “close” button somewhere on the Leo panel for cases like this. If we don't want to clutter the panel, I'd suggest putting the option in the "three dots" drop down menu:

Steps to Reproduce

1. Disable "Show sidebar button" option in Settings --> Appearance
2. Open a new tab (or any tab) and enter a term in the address bar, select Ask Leo option
3. When Leo panel opens, observe that there is no way to close the panel without closing the tab or knowing the keyboard shortcut.

Reproduces how often:

Always

Brave version (brave://version info)

v1.62.156 and higher

Version/Channel Information:

• Can you reproduce this issue with the current release? Yes
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

[aguscruiz]

Added design here
https://www.figma.com/file/m0Gdbf0wtqyfEFGm32VLLc/Leo?type=design&node-id=257%3A12015&mode=design&t=6VKNtNyu2QGM554A-1

[Kchecksfiles]

UPDATE: feature not yet rolled out in official build. [Version 1.63.174 Chromium: 122.0.6261.128]

[DeveloperAlly]

This is the absolutely worst UI.

I am completely removing because of this issue.

I have no idea who thought it was ok not to have a close button