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.

src/cli/verify.ts

@@ -6,13 +6,14 @@
  */
 
 import { hasFatal, validateSite } from '../storage/validate.ts';
+import { SectionStore } from '../storage/sections.ts';
 import { loadCliSite } from './site.ts';
 
 const { config, dataDir } = await loadCliSite();
-const problems = await validateSite({
-  dataDir,
-  sections: config.sections.map((section) => section.slug),
-});
+const sections = new SectionStore({ dataDir, seed: config.sections });
+await sections.load();
+
+const problems = await validateSite({ dataDir, sections });
 
 if (problems.length === 0) {
   console.log(`ok: ${dataDir}`);