SEO: robots.txt, sitemap.xml, canonical, OG/Twitter tags, JSON-LD
(ProfessionalService) on / and /folders/, plus a reused screenshot as
og-image.png. prepare-www.mjs now copies robots/sitemap/og-image into
the build. Also carries pending mobile layout tweaks (toc/prefs bar,
viewport-fit=cover) that were already staged in the working tree.
