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.

docs/adr/0005-sections-come-from-configuration.md

@@ -1,3 +1,5 @@
+**Status**: superseded by [ADR 0007](./0007-categories-are-content.md).
+
 # Sections come from configuration, not from the admin
 
 A Site's Sections are declared in its config file and change only with a redeploy; the admin manages the Menu Sets inside Sections and cannot create, rename or reorder Sections. Sections are few and stable, and keeping them out of the admin removed a whole class of editing, reordering and deletion edge cases from the content model.