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.

Beginne beim GitHub-OAuth-Callback-Handler unter /api/github/oauth/callback und reproduziere die im Issue angezeigte access_denied-URL. Verfolge, wie der Callback derzeit antwortet, und überprüfe dann, dass die Ablehnung eine klare Meldung mit Hinweisen zur Behebung erzeugt und dass die Fehlerdetails protokolliert werden; teste das Wiederholungs- oder Navigationsverhalten, falls implementiert.

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