Bug: Handle GitHub OAuth Access Denied Error

When using GitHub OAuth for authentication in our application, users may encounter an "access_denied" error if they deny our application access, leading to dead blank screen (see screenshot). This error needs to be handled gracefully to provide a good user experience and guide users on how to resolve the issue.

#### Solution:
1. **Error Detection:** Implement error detection logic in the OAuth callback handler to identify the "access_denied" error.

2. **Error Handling:** When the "access_denied" error is detected, display a user-friendly message to inform the user that their access has been denied and provide guidance on how to proceed.

3. **Logging:** Log the error details for debugging and monitoring purposes.

4. **Redirect:** Optionally, provide a redirect mechanism to allow the user to retry the authentication process or navigate back to a previous page.

#### Example Error Details:

- Full URL: `http://localhost:4321/api/github/oauth/callback?error=access_denied&error_description=The+user+has+denied+your+application+access.&error_uri=https%3A%2F%2Fdocs.github.com%2Fapps%2Fmanaging-oauth-apps%2Ftroubleshooting-authorization-request-errors%2F%23access-denied&state=path%3A%2Fsandbox%7Credirect%3Atrue`
- Error: `access_denied`
- Description: `The user has denied your application access.`
- Error URI: [GitHub OAuth Troubleshooting Documentation](https://docs.github.com/apps/managing-oauth-apps/troubleshooting-authorization-request-errors/#access-denied)
- State: `path:/sandbox|redirect:true`

#### Acceptance Criteria:
- Users are presented with a clear message when the "access_denied" error occurs.
- The message includes instructions or a link to resolve the issue.

#### Related Tasks:
- Implement error detection in the OAuth callback handler.
- Design and implement a user-friendly error message for the "access_denied" error.
- Test the error handling mechanism to ensure it works as expected.

#### Additional Notes:
- This issue is crucial for maintaining a positive user experience and ensuring users can easily resolve authentication issues.

Commencez par le gestionnaire de callback GitHub OAuth à l’adresse /api/github/oauth/callback et reproduisez l’URL access_denied affichée dans l’issue. Suivez la manière dont le callback répond actuellement, puis vérifiez que le refus produit un message clair avec des indications pour le résoudre et que les détails de l’erreur sont journalisés ; testez le comportement de nouvelle tentative ou de navigation s’il est implémenté.

Rédigé par le modèle d'indexation à partir du texte de l'issue.