feat: Add preview deployment URL auto-detection using @vercel/related-projects

Summary

When viewing preview deployments, cross-app navigation links (from calculator to website and vice versa) incorrectly go to production URLs instead of the corresponding preview deployment.

Current behavior

Proposed solution

Use Vercel's @vercel/related-projects package to automatically sync preview deployment URLs between the website and calculator projects.

Changes required

1. Install @vercel/related-projects
2. Add relatedProjects config to both vercel.json files with project IDs
3. Update constants.ts to use withRelatedProject():

import { withRelatedProject } from '@vercel/related-projects';

export const WEBSITE_URL = withRelatedProject({
  projectName: 'policyengine-website',
  defaultHost: import.meta.env.VITE_WEBSITE_URL || 'https://policyengine.org',
});

Limitations

• Max 3 related projects can be linked
• Only works for projects in same repository
• CLI deployments not supported

References

• Vercel Monorepos Docs
• @vercel/related-projects changelog

Start by reading constants.ts and both vercel.json files, then review how the current environment URLs are selected. Install @vercel/related-projects and configure the two projects as described. Done means preview deployments link to the corresponding preview app while development and production URLs continue to work.