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.

README.md

@@ -32,7 +32,8 @@ npm run check-site -- https://menu-new.hopsngrains.com
 ## Layout
 
 ```
-src/config.ts             Site config: brand, theme, Sections, limits
+src/config.ts             Site config: brand, theme, seed Sections, limits
+src/storage/sections.ts   Categories: the Site's own list, edited in the admin
 src/storage/manifest.ts   Manifest type, validation, atomic JSON writes
 src/storage/sets.ts       The Menu Set store: create, add Pages, publish, restore, delete
 src/storage/validate.ts   Boot validation: fatal problems and warnings
@@ -43,7 +44,7 @@ src/routes/public.ts      Landing, Section, archive and media routes
 src/routes/admin.ts       Sign in, dashboard, editor, upload, publish, restore, delete
 src/server.ts             buildApp (what tests drive) and main (what the container runs)
 public/                   Hand-written CSS, ~2KB of progressive-enhancement JS, Archivo
-config/                   Per-Site config, one file per Site
+config/                   Per-Site brand, theme, limits and the first category list
 test/                     node:test suites
 docs/plan.md              The agreed plan
 docs/operating.md         Publishing, fixing and rolling back, for whoever operates it