Next.js migration: Vite cleanup (deferred)

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 #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 (#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

Read parent issue #811 and confirm the site is stable and ui-kit integration is complete before starting. Then review the listed Vite scripts, vite.config.mjs, website.html, calculator.html, router files, SPA fallback middleware, and CI configuration. Done means obsolete Vite and router paths are removed, CI no longer builds the Vite app, and the requested metadata improvements are addressed.