Migrate external apps from iframes to Next.js multi-zones

Summary

External interactive apps (WATCA, TAXSIM, Oregon Kicker, CA Wealth Tax, marriage calculator, etc.) are currently embedded via iframes inside a wrapper page. This causes:

• No shared header/footer with the main site (rewrite-proxied apps like Model and API render their own chrome)
• Cross-env linking issues — links inside proxied apps go to production instead of the current environment (#565, #579, #580)
• Iframe-specific UX limitations (no URL bar updates, no back button integration, CSP constraints)

Proposed approach

Convert external apps to Next.js multi-zones — each app is an independent Next.js app that renders full pages (no iframe) under policyengine.org.

What each zone app needs

• assetPrefix in next.config.ts to avoid asset collisions
• Import shared Header/Footer from @policyengine/ui-kit
• Asset rewrites in the main website's next.config.ts
• Cross-zone links use <a> tags (hard navigation), not <Link>

What the main website needs

• Rewrites for each zone's paths AND asset prefix paths
• serverActions.allowedOrigins config if zones use Server Actions

Benefits

• Seamless navigation — no iframe boundary, shared header/footer
• Cross-env linking works — each zone renders its own header with relative links
• Independent deploys per zone
• Each zone can have its own build/framework if needed

Prerequisites

• @policyengine/ui-kit Header/Footer integration (#793)
• Website cutover to production (#817)

Related

• #793 — Extract header/footer into shared package
• #817 — Next.js cutover + cleanup
• #565, #579, #580 — Preview env cross-app linking issues
• #860 — Port Tailwind/shadcn foundation (PR #875)