Redirects the retired GitHub Pages addresses to spreadpaper.app.
SpreadPaper/index.html catches the old project-page path, spreadpaper.github.io/SpreadPaper/, which is still linked from elsewhere on the web. index.html catches the bare domain.
Both carry rel=canonical and a meta refresh, plus a script redirect for browsers. GitHub Pages cannot emit a real 301, so this is a soft redirect — it passes most ranking signal, not all. 404.html is deliberately not used: GitHub serves it with a 404 status, which crawlers ignore.
The site itself lives in spreadpaper/website.