Next.js migration: Vite cleanup (deferred)

[SakshiKekre]

Deferred cleanup tasks from the Vite → Next.js migration (#811). To be done after site is stable and ui-kit is integrated.

Remaining deletions

• Remove app/scripts/generate-prerender.ts (replaced by Next.js SSG)
• Remove app/scripts/generate-sitemap.ts (replaced by website/src/app/sitemap.ts)
• Remove app/scripts/generate-llms-txt.ts (replaced by route handlers in PR Add llms.txt route handlers to Next.js website #905)
• Remove Vite config (vite.config.mjs, website.html, calculator.html)
• Remove React Router (react-router-dom dependency, router files)
• Remove SPA fallback middleware logic
• Update CI to stop building Vite app

Improvements (not regressions)

• Add generateMetadata to home page and research listing for social sharing previews (Next.js migration: middleware + SEO #815)
• Optionally add generateMetadata to supporters and citations pages

Context

Migration audit on 2026-04-02 confirmed all phases are functionally complete. Website and calculator both serving from Next.js in production. These items are safe to defer until ui-kit integration stabilizes shared components.

Parent issue: #811