feat(app): the module registry drives the render (S6b DoD)

Replace App's page-render ternary and the three side-rail page conditionals with
a data-driven lookup: components/moduleRegistry maps each page to a prop-free
content component (+ optional side rail + shell fadeTop), and App renders
PAGE_CONTENT[page]/PAGE_RAIL[page] gated by moduleDef(page).gate (an unreachable
gated page falls back to the feed, as the old ternary did). Content modules and
the two rails are now prop-free: they read `me` via the new useMe() hook and
their module-local state from the scoped providers; FeedPage/MessagesPage are the
only wrappers (they wire the global feed scope / meId). App.tsx 796->431 lines,
zero `page===X` chrome conditionals — closes the Platform epic DoD. (Platform S6b.3)

frontend/src/components/AdminUsers.tsx

@@ -10,6 +10,7 @@ import { Section } from "./ui/form";
 import { useConfirm } from "./ConfirmProvider";
 import Tabs, { usePersistedTab } from "./Tabs";
 import { PageToolbar } from "./PageShell";
+import { useMe } from "../lib/useMe";
 
 // Admin-only user & access management: roles, access requests (the Invite whitelist), and the
 // demo whitelist + reset. Each section is its own tab (persisted across reloads); the Access tab
@@ -17,7 +18,8 @@ import { PageToolbar } from "./PageShell";
 // (The tab key + a "focus the Access tab" helper live in lib/adminUsersTab so callers can
 // pre-select the tab without importing this lazy-loaded page — see that file.)
 
-export default function AdminUsers({ me }: { me: Me }) {
+export default function AdminUsers() {
+  const me = useMe();
   const { t } = useTranslation();
   const [tab, setTab] = usePersistedTab(ADMIN_USERS_TAB_KEY, "roles");
   const tabs = [