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/Sidebar.tsx

@@ -15,6 +15,7 @@ import PanelGroups, { type PanelItem } from "./PanelGroups";
 import ChannelPicker from "./ChannelPicker";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 import { useChannelsActions } from "./ChannelsProvider";
+import { useMe } from "../lib/useMe";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
 const DATE_PRESETS: { days: number; key: string }[] = [
@@ -64,17 +65,16 @@ function TagChip({
 export default function Sidebar({
   layout,
   setLayout,
-  isDemo = false,
   collapsed,
   onToggleCollapse,
 }: {
   layout: PanelLayout;
   setLayout: (l: PanelLayout) => void;
-  isDemo?: boolean;
   collapsed: boolean;
   onToggleCollapse: () => void;
 }) {
   const { t } = useTranslation();
+  const isDemo = useMe().is_demo;
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
   // "Focus this channel in the manager" (from the tag manager here) lives in ChannelsProvider.