Add pop-out button for chat window

[Copilot]

When the chat is embedded in documentation pages, users can't view docs and chat side-by-side. This adds a button to pop the chat out into its own window.

Changes

• Added external-link icon button to chat header toolbar (next to GitHub icon)
• Opens current chat URL in a 500x700 popup window via window.open()
• Includes noopener,noreferrer for security

window.open(
  window.location.href,
  "_blank",
  "width=500,height=700,menubar=no,toolbar=no,location=no,status=no,noopener,noreferrer"
);

Screenshot

Original prompt

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This section details on the original issue you should resolve

<issue_title>button to pop out chat</issue_title>
<issue_description>As in this preview:

bids-standard/bids-specification#2254

The problem is that the content of this window does not follow page changes. You can improve this a bit by making all the links open new windows, but then it requires a lot of flipping back and forth between the docs and the chat. It would be nice to have a button that could pop this chat out into its own window so it can be viewed side-by-side with the documentation.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes button to pop out chat #23

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