move GithubCallback() to client

[arnavsurve]

Logic should be handled in the client at localhost:9998/oauth2/callback. After authentication, the client should be redirected to this endpoint with oauth code and state. Handle oauth flow here and store JWT and username in DB. Also POST to github account creation endpoint in the backend. (See other sub issues)

[linear]

TAS-53 move GithubCallback() to client