Fix/auth google callback redirect

[KGFCH2]

Related Issue

Closes #131

Summary

Updates Google OAuth callback redirects to use the configurable frontend URL instead of a hardcoded localhost address.

Changes Made

• Added CLIENT_URL resolved from process.env.FRONTEND_URL
• Updated failure redirect and callback success redirect in server/routes/authRoutes.js

Testing

• Verified backend route compiles
• Confirmed redirect base can be configured through FRONTEND_URL
• Ensured no hardcoded localhost remains

Impact

Improves OAuth compatibility across staging and production deployments.

Checklist

• Code follows project standards
• Tested locally
• No unrelated changes included
• Environment configuration updated