Click "Back" after login would see error for invalid page

## Step to reproduce

In a web app using authgear to login.

1. Trigger login
2. Login on Authgear, redirected to the web app
3. Navigate "Back", see invalid page error

## Solution

This error message is expected for the oauth flow because the previous flow is already completed. However this is frustrating for the user because they got stuck in that page after accidentally back.

Alternatively, by manipulating the history stack, see if we can achieve the following:
- When clicking the back button for 1st time, goes to to the redirect page after login, so the user is redirected to the app again.
- If clicking the back button twice quickly, go to the page before login.

No file or test is named. Start by tracing the login redirect and browser history behavior in the web app flow, then determine whether the first Back action can return to the post-login redirect while two rapid Back actions reach the page before login; done means invalid-page errors no longer trap users after login.

Written by the indexing model from the issue text.