Static/docs/example routes are served as <route>/index.html and nginx
301-redirects the slash-less form to add the trailing slash. The sitemap
generator, the prerender canonical/og:url, and the client useSEO canonical
all emitted the slash-LESS form, so every sitemap URL was fetched as a
redirect (filed under 'Page with redirect' in Search Console) and each
canonical pointed at a redirecting URL.
Align all three to the trailing-slash form so sitemap URL == canonical ==
served URL == 200, with no redirect hop.
- generate-sitemap.mjs: append '/' to every <loc> (root stays '/')
- prerender-seo.mjs: withSlash() on canonical + og:url (routes + examples)
- useSEO.ts: withTrailingSlash() on canonical + og:url (covers dynamic
project pages too)