feat(frontend): add environment-based API configuration (fixes #156)

[Diwakar-odds]

📝 Description

This PR implements environment-based API configuration (dev/staging/prod) for the frontend application.

Specifically, this PR:

• Creates a central config.ts file that maps development, staging, and production environments to their respective backend URLs.
• Updates all frontend components and hooks (useAuth.ts, useCompareVersions.ts, App.tsx, AccountSettingsModal.tsx) to import and use the central BACKEND_URL instead of hardcoding http://127.0.0.1:8000.
• Allows overriding the configuration using the explicit VITE_BACKEND_URL environment variable if defined.

🔗 Related Issue

Closes #156

✅ Type of Change

• 🐞 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 📚 Documentation update
• 💅 UI/UX improvement
• ♻️ Refactor (no functional change)
• ⚡ Performance improvement

🧪 How Has This Been Tested?

• Frontend builds (npm run build) and lints (npm run lint) clean
• Backend tests pass (python manage.py test analyzer)
• Manually tested in the browser / API client

📸 Screenshots (if applicable)

📋 Checklist

• My code follows the project's style and conventions
• I have linked the related issue above
• I have read the Code of Conduct

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[Diwakar-odds]

I have completed this task. This PR introduces environment-based API configuration (dev/staging/prod) for the frontend application.

[Muskankr]

@Diwakar-odds , Please resolve merge conflicts

[Muskankr]

@Diwakar-odds , Thanks for your contribution! 🎉 If you enjoyed contributing to this project, consider giving the repository a ⭐. It really helps the project grow and motivates future development.