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/DownloadCenter.tsx

@@ -27,7 +27,8 @@ const VideoEditor = lazy(() => import("./VideoEditor"));
 const ShareDialog = lazy(() => import("./ShareDialog"));
 import { useConfirm } from "./ConfirmProvider";
 import { useLiveQuery } from "../lib/useLiveQuery";
-import { api, type DownloadJob, type Me } from "../lib/api";
+import { useMe } from "../lib/useMe";
+import { api, type DownloadJob } from "../lib/api";
 import { invalidateDownloads } from "../lib/downloads";
 import { notify } from "../lib/notifications";
 import { formatBytes, formatDuration, formatEta, formatSpeed } from "../lib/format";
@@ -539,7 +540,8 @@ function AdminSystem() {
 
 // --- Main -----------------------------------------------------------------------------------
 
-export default function DownloadCenter({ me }: { me: Me }) {
+export default function DownloadCenter() {
+  const me = useMe();
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();