Next.js migration: cutover + cleanup

[MaxGhenis]

Part of #811

Goal

Cut over from Vite to Next.js and clean up the old build system.

Tasks

• Verify all pages render identically to current production
• Run visual regression tests against current site
• Switch Vercel project from Vite build to Next.js build
• Remove Vite config (vite.config.mjs, website.html, calculator.html)
• Remove React Router (react-router-dom dependency, router files)
• Remove build scripts replaced by Next.js (generate-prerender.ts, generate-sitemap.ts, generate-llms-txt.ts)
• Remove SPA fallback middleware logic (no longer needed with SSR/SSG)
• Update CI workflows for Next.js build
• Update CLAUDE.md and development docs
• Monitor production for errors post-cutover
• Update Storybook config if needed

Rollback plan

• Keep Vite build artifacts in a branch for quick rollback
• Use Vercel instant rollback if issues arise post-deploy

Notes

• Don't rush this — run both builds in parallel for a period if possible
• Consider a staged cutover: website first, calculator second

[SakshiKekre]

Cutover complete — both policyengine.org (website) and app.policyengine.org (calculator) serving from Next.js. Post-cutover fixes applied (PR #885, #883, #878). Vite cleanup deferred to #907 (after ui-kit integration).