Urgent Flutter OAuth Fix

My production Flutter app cannot complete its OAuth login flow after a recent backend change. The issue sits squarely in the API integration layer: the request returns a 401 even though the same credentials succeed from Postman. I need a Flutter specialist who can jump into a remote session (TeamViewer or AnyDesk) and resolve this today, billing by the hour.

You will be working on my machine because the full codebase cannot leave our network. Once connected, you’ll trace the handshake, inspect headers, review the interceptor that stores tokens, and adjust any redirect or callback handling that is off-spec. I will be online the whole time to supply client secrets, test accounts, or backend logs you may need.

Acceptance criteria:
• OAuth sign-in completes and lands on our home screen without errors.
• Access and refresh tokens are saved locally and automatically refreshed before expiry.
• Failed logins surface the server-supplied error message in our current Snackbar pattern.
• A short summary of the changes (files touched and why) is recorded in a shared doc for my later reference.

The build already runs on stable Flutter 3.x with the http and oauth2 packages; no version bumps are required unless you find that’s the root cause. If you’re confident troubleshooting live, let’s set up a session right away so we can have the fix deployed within the day.