docs: add WorkOS CORS configuration to deployment guide

[hhwjsw711]

Summary

Adds missing WorkOS CORS configuration step to the deployment documentation to prevent authentication errors in production deployments.

Problem

When deploying OpenSync to Vercel/Netlify using the one-click deploy, users encounter CORS errors during authentication:

Access to fetch at 'https://api.workos.com/user_management/authenticate' 
from origin 'https://your-app.vercel.app' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource.

This happens because:

• The app uses devMode={true} in AuthKit to avoid third-party cookie issues
• In dev mode, AuthKit makes client-side requests to WorkOS API
• WorkOS requires the deployed URL to be added to allowed CORS origins
• This step was missing from the deployment documentation

Changes

• Step 5 added to WorkOS setup: Clear instructions on configuring CORS in WorkOS Dashboard
• New troubleshooting section: "CORS error: Access-Control-Allow-Origin header missing" with step-by-step resolution
• Updated existing troubleshooting: Added CORS verification to "Login redirects but user stays on login page"

Testing

Verified that after following the updated documentation:

1. CORS configuration in WorkOS Dashboard resolves the authentication error
2. Users can successfully log in after deployment
3. Convex user table is properly populated

Related

This issue affects all production deployments using the one-click deploy feature and was discovered during a real deployment scenario.

🤖 Generated with Claude Code

[netlify]

👷 Deploy request for opensyncsessions pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	007a970