test(e2e): a plain member's shell is held end to end, and a page it cannot open lands on the feed

The suite ran only as an admin, so nothing end to end held what a member sees. A second seeded
account, a member with its own session, now walks every rail page (no admin entry, no admin call,
no error state) and follows links to the admin pages.

It found one: a member's link to an admin page showed the feed with no rail row current, and the
address bar kept naming the admin page — only the content fell back. The navigation state now
derives the page on screen through each module's gate (reachablePage), so the rail, the content,
the tab title and the address bar agree, on a link, a Back, and a role taken away mid-session.

frontend/src/lib/modules.ts

@@ -183,3 +183,11 @@ export const SYSTEM_PAGES: readonly Page[] = MODULES.filter((m) => m.system).map
 export function moduleOrder(me: Me): Page[] {
   return MODULES.filter((m) => !m.gate || m.gate(me)).map((m) => m.id);
 }
+
+/** The page this account is shown for `page`: itself when its gate lets the account in, else the
+ *  feed (a link to an admin page, a Back onto one, an admin role taken away mid-session). Signed
+ *  out, no page renders, so nothing is moved. */
+export function reachablePage(page: Page, me: Me | null | undefined): Page {
+  const gate = moduleDef(page).gate;
+  return !me || !gate || gate(me) ? page : "feed";
+}