[Bug] Google OAuth login fails intermittently (localhost:9876 returns 500 / no request at all), while GitHub OAuth works fine

## Description

Google sign-in always fails with `Firebase: Error (auth/popup-blocked)`, then redirects to browser and shows `Sign-in failed - fetch failed`. GitHub sign-in failed to with `Firebase: Error (auth/popup-blocked)`, but it can be opened and signed in finally, fine on the same machine.

## Environment

- OS: Windows 11
- Browser: tested Chrome and Edge, same result
- Proxy: enabled, localhost already excluded from proxy

## Steps to Reproduce

1. Click "Log in with Google"
2. See `auth/popup-blocked` error
3. Redirects to browser at `http://localhost:9876/?n=...`
4. Result is inconsistent:
- sometimes port 9876 isn't listening at all (connection refused)
- sometimes it returns 500 Internal Server Error
5. Ends with "Sign-in failed - fetch failed"

## Notes

- GitHub login opens a small companion popup window during the flow. Google login never opens this window — seems like the local callback listener isn't starting properly for Google specifically.
- I have credits tied to my Google account and can't sign in to access them.

## Screenshots

Start by tracing the Google OAuth flow and its localhost:9876 callback listener, then compare it with the working GitHub popup flow. Reproduce the intermittent connection-refused and 500 responses in Chrome or Edge; done means Google sign-in reliably completes without auth/popup-blocked or fetch-failed errors.

Written by the indexing model from the issue text.