fix exporting window.VITE_BACKEND_URL if isBackendSet

[tiero]

This fixes problem found doing the #338 PR. I did it here also in isolation in case that PR will not be merged.

[what-the-diff]

PR Summary

• Enable broader backend URL configuration
  The code has been updated to allow for a more flexible configuration of the backend URL. Previously, it only accepted URLs from a specific source (import.meta.env.VITE_BACKEND_URL). But now, it also looks for URLs from a global variable (VITE_BACKEND_URL). This change enhances the flexibility in setting up the application's backend connection, accommodating various deployment scenarios.