fix(examples): handle OAuth access_denied in auth-headless example

[SpencerJung]

fix(examples): handle OAuth access_denied in auth-headless example

Description

When a user cancels GitHub OAuth login by clicking "Cancel" on the GitHub authorization screen, the callback URL contains error=access_denied. Previously, the authProvider did not handle this case, resulting in a generic "No user information from OAuth provider" 401 Unauthorized error.

This PR adds handling for the access_denied error in the auth-headless example's authProvider check method. Now when a user cancels OAuth, they see a friendly error message: "Login cancelled by user" / "Authorization denied".

Changes

• Updated examples/auth-headless/src/App.tsx
• Added OAuth cancellation detection in authProvider.check()
• Returns user-friendly error message when error=access_denied is detected in URL

Testing

1. Navigate to login page
2. Click "Sign in with GitHub"
3. On GitHub authorization screen, click "Cancel"
4. Verify that a user-friendly error message is displayed instead of 401

Related Issue

Closes #7407

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Latest commit: 9383f92

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