Copilot login opens the bare device-flow URL instead of verification_uri_complete

[skhe]

When logging into GitHub Copilot from CodexBar, the browser opens the generic device-flow verification page, but CodexBar does not use verification_uri_complete from the device code response.

That makes the login flow fragile on the desktop side, because users may not see an obvious pre-filled device code and can get stuck even though CodexBar already received the user_code.

Observed behavior:

• CodexBar requests a device code
• It copies user_code to the clipboard
• It opens verification_uri
• On the GitHub/Copilot side, the user may land on a page where the code is not visible or not obviously applied

Expected behavior:

• If verification_uri_complete is present, CodexBar should open that URL instead
• Fall back to verification_uri only when the complete URL is absent

I prepared a small fix for this by decoding verification_uri_complete and preferring it when opening the browser.