Automatic navigational cards on index pages, replacing the sidebar menu.

docs/backend.md

@@ -26,7 +26,9 @@ The shared page layout as an html5tagger `Template` with placeholders (`Title`,
 
 Content pages get SEO/social meta (description, canonical link, Open Graph + twitter card) from heuristics over the rendered article: the description is the first paragraph's text, the share image prefers a `{.hero}`-classed image, then the first raster `<img>`, then the first SVG; the first `<video>` yields `og:video`; URLs are made absolute with the site origin (`Data.site_url` — learned from admin browsers reporting their `location.origin` via `POST /_api/site-url`, correct even behind reverse proxies; until learned, the request's own base URL is the fallback); `article:published/modified_time` come from `Node.created`/`modified`. If the markdown contains its own h1, the page title is NOT rendered as an additional h1 (it still supplies `<title>` and nav labels).
 
-The navbar holds top-level items only; the current section's subitems go to a left `#sidebar` as a nested list (the section's direct children plain, deeper levels indented with article-list-style markers), which is rendered when the section offers at least two published items, or exactly one while viewing anything other than that only page — the section index, a 404, a grandchild (so those pages can reach the child), and also on that only page itself when it has published children of its own; no aside element at all on the front page, leaf pages and the sole childless page of a one-page section. Also, category labels are nodes without content — None *or* empty markdown — and their nav links point at their first child page. Dynamic regions have stable ids (`#page-banner`, `#nav`, `#sidebar`, `#main`) for fetch-navigation swaps (`#sidebar` may be absent on either side of a swap).
+The navbar holds top-level items only; the current section's subitems go to a left `#sidebar` as a nested list (the section's direct children plain, deeper levels indented with article-list-style markers), rendered only from the second level down — main-level pages list their children as cards after the content instead. Below that, the sidebar renders when the section offers at least two published items, or exactly one while viewing anything other than that only page — the section index, a 404, a grandchild (so those pages can reach the child), and also on that only page itself when it has published children of its own; no aside element at all on the front page, main-level pages, leaf pages and the sole childless page of a one-page section. Also, category labels are nodes without content — None *or* empty markdown — and their nav links point at their first child page. Dynamic regions have stable ids (`#page-banner`, `#nav`, `#sidebar`, `#main`) for fetch-navigation swaps (`#sidebar` may be absent on either side of a swap).
+
+Any page with published children — a category page — lists them as a card grid (`nav.cards`) after the markdown content, as does the content-less category 404. Each card links to the child page (a content-less child to its first leaf) and shows the child's share image (the same hero → first raster → first SVG heuristics as `og:image`) as a full-card cover with the title overlaid.
 
 ## `seed.py`