extensions: fix command palette accessibility

Created by: tjkandala

Part of #36751 (closed) Closes #36858 (closed), #36859 (closed), and #36861 (closed)

• Allow <CommandListPopoverButton> to be a button. For unknown reasons, it was a <span> by default. This makes it focusable, and I didn't observe visual regressions on web (didn't change browser extension/native integration usage).
• Capture active element on open in order to restore focus on close.
• Limit command palette width on mobile.

Test plan

• Existing automated tests pass
• Ensure focus works through manual testing
  • Tab through global navbar
    • Observe that the command palette icon is now focusable.
    • 
  • Focus some element on the page. Activate the command palette with the ctrl + p shortcut.
    • Execute an action or press escape
    • Observe that focus returns to the previously focused element

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.