Next.js migration: external embeds + rewrites

[MaxGhenis]

Part of #811

Goal

Migrate external app embeds and Vercel rewrites to the Next.js app.

Tasks

• Port Vercel rewrites from vercel.json to next.config.ts rewrites:
  • /slides/* → policyengine-slides
  • /_tracker/* → Modal state legislative tracker
  • /us/api/* → household-api-docs
  • /us/watca/* → Working Americans Tax Cut calculator
  • /us/taxsim/* → TaxSim calculator
  • /plugin-blog/* → GitHub Pages plugin blog
• Port iframe embed pages:
  • /:countryId/ai-inequality → policyengine.github.io/ai-inequality
  • /:countryId/2025-year-in-review → policyengine.github.io/2025-year-in-review
• Port /us/model page (policyengine-model iframe)
• Port legacy redirects (/blog/* → /research/*)
• Verify all embedded content loads correctly
• Update CI embed URL checker for new route structure

Notes

• next.config.ts rewrites() is the direct equivalent of vercel.json rewrites
• Redirects can use next.config.ts redirects() or middleware
• Iframe pages are simple — just need the component ported to a Next.js route

[SakshiKekre]

All rewrites ported to website/next.config.ts (slides, tracker, model, API docs, watca, KYPA, Oregon Kicker, taxsim, CA wealth tax, plugin-blog). Iframe pages ported (AI inequality, 2025 year in review, ads dashboard, org logos). Legacy /blog → /research redirects working. All embeds verified working in production.