Add SEO baseline + rebuild www/ (mobile toc/prefs polish, responsive viewport)

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.

public/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://levkin.ca/sitemap.xml

public/sitemap.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://levkin.ca/</loc>
+    <changefreq>monthly</changefreq>
+    <priority>1.0</priority>
+  </url>
+  <url>
+    <loc>https://levkin.ca/folders/</loc>
+    <changefreq>monthly</changefreq>
+    <priority>0.8</priority>
+  </url>
+</urlset>