GitHub authentication

[arnabnandy7]

Summary

Add GitHub authentication so users can connect their GitHub account to OpenIssue.dev.

Motivation

Authentication unlocks personalized features such as saved searches, recommendations, contribution history, and alerts.

Proposed Scope

• Add “Sign in with GitHub” support.
• Store basic authenticated user profile data.
• Keep unauthenticated search usable.
• Use the authenticated GitHub token for higher API rate limits where appropriate.

Acceptance Criteria

• Users can sign in and sign out with GitHub.
• Auth state is reflected in the UI.
• Existing search behavior still works without login.
• GitHub API calls can optionally use the authenticated token.