Static pages that fetch local JSON break over file://

If a static landing page uses fetch() to load a sibling JSON file, browsers block it on the file:// protocol, so put the serve hint (e.g. python3 -m http.server) directly in the fetch catch-branch UI instead of only in docs. Two other cheap wins: set data-theme on documentElement in a tiny head script (localStorage fallback to prefers-color-scheme) before CSS paints to avoid dark-mode flash, and validate the HTML with python3 html.parser by checking tag balance against the void-element list, duplicate ids, and that every #anchor href resolves to a real id.