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/lib/useMe.ts

@@ -0,0 +1,13 @@
+import { useQuery } from "@tanstack/react-query";
+import { api, type Me } from "./api";
+
+// The signed-in account, read from the ["me"] query cache. App owns the FETCHING query (gated on
+// setup state) and renders modules only once it has resolved to a real user — so anywhere inside the
+// authenticated module tree the cache is always populated. `enabled:false` makes this a passive
+// observer (no second fetch, which could 503 in setup mode) that still re-renders when the cache
+// updates (login, the 60s refetch, a Google-link round-trip). Lets modules read `me` from a hook
+// instead of having it prop-drilled through App.
+export function useMe(): Me {
+  const { data } = useQuery({ queryKey: ["me"], queryFn: api.me, enabled: false });
+  return data as Me;
+}