Hero redesign + lateral sidebar — umbrella #149 closed

The KDS landing got its visual identity update. Hero now demonstrates the project's core thesis in-place: a `spec → render` panel showing a snippet of `specs/icons/products.kmd` next to the Koder Design icon it produces. The top navigation moved to a lateral sidebar with collapsible groupings (Fundamentos / Composição / Comportamento / Meta / Tools) and active-state highlighting. Plus seven follow-up bug fixes from the parade: theme persistence across pages, four inline scripts that were silently broken in prod (templ 0.3.x quirk), sidebar groups closed-state, ToC sticky topbar clearance, coverage matrix table semantics, and the page-toc scrollbar polish.