Add immersive-safe 3D device auth prompt for headless Google login

[Copilot]

Headless Google device auth relied on OnGUI, which is not visible in immersive/XR flows without a 2D layer. This change adds an in-world prompt so users in immersive mode can still read the verification URL and device code.

• Problem addressed

  • Preserve headless device-auth usability when immersive mode cannot render the existing 2D dialog.

• Runtime auth UX updates (ArenaMqttClient)

  • Added a world-space TextMeshPro prompt that is shown only while showDeviceAuthWindow is active.
  • Prompt content mirrors device-auth instructions (verification URL + user code).
  • Prompt is camera-relative (kept in front of the user) so it remains readable in XR/immersive contexts.

• Lifecycle + safety

  • Added explicit prompt teardown when auth prompt state ends and on object destroy.
  • Added defensive cleanup if prompt component creation fails.

if (showDeviceAuthWindow)
{
    UpdateDeviceAuthPrompt3D(); // shows URL + code in world space
}
else if (deviceAuthPrompt3D != null)
{
    DestroyDeviceAuthPrompt3D();
}