Fix bug with redirect url

[amrit110]

This pull request updates the OAuth callback route to ensure all redirects use a consistent base URL, which can be configured via an environment variable. This change makes redirect behavior more predictable, especially in environments where the app may be accessed from different domains or behind proxies.

Redirect handling improvements:

• Added a getBaseUrl helper function to determine the base URL for redirects, using the NEXT_PUBLIC_APP_URL environment variable if set, or falling back to the request's origin. All redirects now use this base URL for consistency.
• Updated all redirect responses in the OAuth callback handler to use the new base URL, including error cases and after successful authentication. This affects redirects for errors such as invalid callback, invalid state, missing verifier, token exchange failure, unauthorized domain, and general authentication failure. [1] [2] [3]