Let the owner manage categories, and give the menus a shelf

Categories were baked into config/<site>.site.json, so adding one meant a
rebuild and a redeploy. They are now data: data/sections.json, seeded from the
config the first time, and managed in the admin (add, rename, reorder, remove).
A category's web address is fixed when it is created because that is what a QR
code points at, and a category cannot be removed while it still holds menus.

The landing and the top bar were the ugliest parts. The landing was a column of
hairlines with tiny meta — a broadsheet, and the paper that makes this app worth
looking at was nowhere on it. It is now a shelf: each category is a row whose
right edge is a sliver of that menu's first page, bleeding off the screen, with
the shelf edge in brass and the name in heavy tight Archivo. The bar was a row
of teal pills that would not survive more categories: it now names the menu you
are reading and opens a sheet listing the rest, each with its own paper sliver,
and the current one is the only teal thing on screen. The sheet is a link to the
landing page, so navigating still works without the script.

Also fixes the category manager's own layout, which I shipped unstyled, and the
markup contract the route tests described.

public/admin.css

@@ -320,3 +320,60 @@
 .login input {
   width: 100%;
 }
+
+/* ---------- Categories ---------- */
+
+.catlist {
+  margin: 0.4rem 0 0;
+  padding: 0;
+  list-style: none;
+}
+
+.catrow {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.4rem 0.6rem;
+  padding: 0.5rem 0;
+  border-bottom: 1px solid var(--hairline-soft);
+}
+
+.catrow__name {
+  display: flex;
+  flex: 1 1 13rem;
+  gap: 0.4rem;
+  min-width: 0;
+}
+
+.catrow__name input {
+  flex: 1 1 auto;
+  min-width: 0;
+  padding: 0.4rem 0.5rem;
+  border: 1px solid var(--hairline);
+  background: var(--steel);
+  color: var(--ink);
+  font: inherit;
+  font-size: 0.95rem;
+}
+
+.catrow__url {
+  color: var(--ink-dim);
+  font-size: 0.8rem;
+  font-variant-numeric: tabular-nums;
+  white-space: nowrap;
+}
+
+.catrow__actions {
+  display: flex;
+  gap: 0.25rem;
+  margin-left: auto;
+}
+
+.fold--categories > summary {
+  color: var(--ink);
+  font-size: 0.95rem;
+}
+
+.fold--categories .muted {
+  margin: 0.5rem 0 0;
+}