feat: embed docs landing in floofy-sun and CI build for floofy.exe

- server: embed docs/index.html and serve it on lab.<root> so the docs
  page works even if DNS is still propagating to GitHub Pages
- server: reserve subdomains (lab, floofy, www, api, mail, ...) — refuse
  friendly slugs claiming them and short-circuit routing for them
- docs: add download button pointing to releases/latest/download/floofy.exe
- ci: workflow that builds floofy.exe on windows-latest, uploads artifact,
  and publishes a rolling 'latest' release on each push to main (plus a
  tagged release on v* tags)