Feature: GitHub OAuth Sign up / Sign in (parity with Google OAuth)

[prayagupa23]

🚀 Pull Request Checklist
[x] ✅ My issue is assigned to me, and I have not taken up another task simultaneously.
[x] 🔁 I have pulled the latest changes from the main branch.
[x] 🧪 My code is tested and does not break existing functionality.
[ ] 📚 I have added/updated documentation wherever necessary.
[x] 🧹 My code follows the project’s coding standards.
[x] ✍️ My commits are clear and meaningful.
[x] 🧾 I have linked the issue this PR addresses with Closes #<issue_number>.

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📌 Related Issue
Closes #41

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🧠 Description
Adds GitHub OAuth authentication alongside the existing Google flow. Authenticated users are issued a JWT on successful GitHub login/signup and are redirected to /dashboard?token=..., where the token is stored for subsequent API calls. Includes a first-time signup fix to ensure the dashboard does not bounce to Login on the initial redirect.

✅ Type of Change
[ ] 🐞 Bug fix
[x] 💡 Feature
[ ] 🧹 Code cleanup/refactor
[ ] 🧪 Test cases
[ ] 📚 Docs update

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📝 Additional Notes (Optional)

• Backend env required: GITHUB_CLIENT_ID, GITHUB_CLIENT_SECRET, GITHUB_CALLBACK_URL, CLIENT_URL, JWT_SECRET, MONGO_URI.
• Frontend env: VITE_API_URL.
• Non‑breaking changes; Google and email/password auth unaffected.

@Annanyatiwary4

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[github-actions]

🙌 Thanks @prayagupa23 for raising this PR!
Our team will review it soon ⏳ — meanwhile, please make sure your PR follows our contributing guidelines 👉 Contributing Guidelines 🎉

[github-actions]

🎉🎉 Thank you for your contribution! Your PR #184 has been merged! 🎉🎉