website: prerender to static HTML for no-JS crawlers/agents

[okcodes]

Summary

• Adds a build-time prerender step so dist/index.html's #root contains real markup instead of shipping empty for client React to fill in — fixes crawlers/AI agents that don't execute JS getting no content.
• Chose this over migrating to Next.js: the site deploys to GitHub Pages (static hosting only, actions/deploy-pages), so Next's actual SSR/ISR advantages need a Node/edge runtime that isn't available here — it would run in output: 'export' mode anyway, i.e. the same static HTML Vite already produces, for a single-page site with no router.
• website/src/entry-server.tsx (renderToString(<App/>)) is built with a separate vite.ssr.config.ts (only the react() plugin — virtualConsoleVitePlugin assumes an HTML build target and breaks otherwise), then website/scripts/prerender.mjs inlines the output into dist/index.html and deletes the intermediate dist-ssr/.
• website/src/main.tsx now hydrates when #root already has content (build/preview) and falls back to a normal client render when empty (vite dev, unprerendered).
• No changes needed to release-website.yml — pnpm run build still ends with static files in website/dist.
• Full reasoning recorded in DECISIONS.md.

Test plan

• pnpm run build in website/ succeeds; dist-ssr/ is created then removed.
• curl of the built dist/ output (via a plain static file server, no JS execution) shows real page copy ("Drop it in", "Try it live", hero text) inside #root.
• pnpm run typecheck and pnpm run lint (oxlint) pass with no new warnings.
• Loaded the built site in a browser — no console errors, no hydration mismatch warnings.
• Reviewer: run pnpm run dev locally and confirm normal client-render still works (unprerendered #root in dev is expected/by design).

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