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.

vite.config.js

@@ -2,6 +2,11 @@ import { resolve } from 'path';
 import { defineConfig } from 'vite';
 
 export default defineConfig({
+  server: {
+    host: true,
+    port: 5175,
+    strictPort: true,
+  },
   build: {
     rollupOptions: {
       input: {