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.

CONTEXT.md

@@ -11,8 +11,10 @@ One restaurant's customer-facing menu deployment, with its own brand, Sections a
 _Avoid_: brand, tenant, deployment, instance
 
 **Section**:
-A top-level category of menu content that customers choose between, such as Food, Bar or What's On.
-_Avoid_: type, category, tab, folder
+A top-level category of menu content that customers choose between, such as Food, Bar or What's On. Sections are content: the owner adds and renames them in the admin.
+_Avoid_: type, tab, folder
+
+The admin, and the owner, say **category** where the code and this glossary say Section. The two words mean the same thing; the code keeps Section so the storage layout, routes and manifests stay consistent.
 
 **Menu Set**:
 One complete set of Pages published together for a Section, with a label and an optional valid-until date.