SeoHead.astro canonical tag as defense-in-depth against redirect bugs

src/components/general/SeoHead.astro builds permalink from Astro.url.pathname (not Astro.site) for og:url/twitter:url — correct at build time since Astro defaults to build.format: "directory", generating trailing-slash paths. Added <link rel="canonical" href={permalink} /> (line ~16) as defense-in-depth against upstream infra bugs (GitHub Pages/Cloudflare redirect issues, see related gotcha) that can serve broken previews for no-slash URLs even though the repo's own HTML is always correct.