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/Channels.tsx

@@ -15,7 +15,7 @@ import {
   UserMinus,
   X,
 } from "lucide-react";
-import { api, type ManagedChannel, type Me, type Tag } from "../lib/api";
+import { api, type ManagedChannel, type Tag } from "../lib/api";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { accountKey, LS } from "../lib/storage";
 import { useDismiss } from "../lib/useDismiss";
@@ -34,6 +34,7 @@ import { useChannels, useChannelsActions } from "./ChannelsProvider";
 import { useNavigationActions } from "./NavigationProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 import { useWizard } from "./WizardProvider";
+import { useMe } from "../lib/useMe";
 
 export type ChannelsView = "subscribed" | "discovery" | "blocked";
 
@@ -46,7 +47,8 @@ const STATUS_FILTERS: { id: ChannelStatusFilter; labelKey: string }[] = [
   { id: "hidden", labelKey: "channels.filters.hidden" },
 ];
 
-export default function Channels({ me }: { me: Me }) {
+export default function Channels() {
+  const me = useMe();
   const canWrite = me.can_write;
   const isAdmin = me.role === "admin";
   // "Connect YouTube" onboarding lives in WizardProvider (a notify action / a mutation-error CTA).