the URL says which page, not which row

`?page=about` instead of `?page=2`. A URL that names the page can be read, sent to somebody, and still
means the right thing after a database is restored from a dump with different ids — which happened
once in this project already, earlier today.

Home needs a word, because its slug is empty by design: that is the check constraint, and it is what
makes `/` fall out of joining a slug to the root. An empty string is no use in a query parameter that
has to name a page unambiguously, so the admin says `home` — and the site's API now REFUSES that slug,
which is what makes the mapping total in both directions rather than merely usual.

One reserved address costs nothing. Anybody who wanted /home wanted the front page, which they have.

One place maps a page to its URL value and back — pageParam and pageFromParam — so the nav, the detail
panel and the builder cannot disagree about what `?page=` means.

`record` is deleted on every navigation. It belonged to the per-page record panels, which are
unreachable now; nothing reads it, so it was not a bug, but every URL anybody copied carried a
parameter naming a row in a section that no longer exists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/usePages.ts

@@ -41,3 +41,22 @@ export const usePages = () => {
 
   return { pages, announce };
 };
+
+/**
+ * The value `?page=` carries, and how to read it back.
+ *
+ * The SLUG rather than the id, so a URL says which page it is: `?page=about` can be read, sent to
+ * somebody, and still means the right thing after a database is restored from a dump with different
+ * ids — which happened once already in this project.
+ *
+ * Home is the exception and needs a word, because its slug is empty by design. `home` is reserved by
+ * the site's API, so no page can take that address and the mapping is total in both directions.
+ */
+export const HOME_PARAM = 'home';
+
+export const pageParam = (page: Page): string => page.slug || HOME_PARAM;
+
+export const pageFromParam = (pages: Page[] | null, value: string | null): Page | null => {
+  if (!pages || !value) return null;
+  return pages.find((page) => pageParam(page) === value) ?? null;
+};