feat(auth): improve GitHub sign-in failure feedback with clear inline guidance (fixes #1529)

[ishwarc04]

Linked Issue

Fixes #1529

Problem

When GitHub authentication failed, users were redirected to the sign-in page without a clear explanation.
This made local setup and OAuth troubleshooting confusing.

What I changed

• Added a dedicated auth error utility to normalize and map auth error codes to user-friendly messages.
• Improved the sign-in page error banner content for GitHub auth failures.
• Added practical next-step guidance in the UI (OAuth credentials and callback URL checks).
• Preserved stale-error cleanup in the URL while keeping banner display behavior reliable.
• Added focused unit tests for auth error normalization and fallback mapping.
• Added a contributor summary file for issue tracking and review context.

Files changed

• src/lib/auth-error-message.ts
• src/app/auth/signin/page.tsx
• test/auth-error-message.test.ts
• ISSUE_1529_SUMMARY.md

Validation

• Ran app locally with development server.
• Ran focused tests for new logic:
  • npm run test -- auth-error-message.test.ts
• Test coverage added for:
  • empty and malformed auth error input
  • URL-decoded error handling
  • known and unknown error-code mapping

Impact

• Better user experience during GitHub auth failures.
• Faster troubleshooting for contributors during local OAuth setup.
• Clearer and actionable feedback instead of silent redirect confusion.

Notes

• jsdom was added to devDependencies to support test environment requirements during Vitest execution.

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