implemented the google OAuth authentication

[saurabh19304]

Summary

Added Google OAuth sign-in support alongside existing GitHub login with a clean dedicated sign-in page.

Closes #122

Type of Change

• New feature google authentication with better UI fix

Changes Made

• Updated src/lib/auth.ts to include Google Provider
• Updated src/app/page.tsx with a single "Sign In to Continue" button
• Created new src/app/signin/page.tsx with both GitHub and Google sign-in options
• Added proper environment variables support for Google OAuth

How to Test

1. Go to homepage → Click "Sign In to Continue"
2. On sign-in page, test both:
   • Sign in with GitHub (should still work)
   • Sign in with Google (new)
3. Verify successful redirect to dashboard after login
4. Check that existing functionality is not broken

Checklist

• Google login works

• GitHub login still works
• No TypeScript errors
• npm run lint passes
• Self-reviewed the code

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Ready for review.

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