refactor(shell): channel page is a PageShell config, not a 2nd scroller

The channel page rendered its own PageScroller as a separate App branch —
the source of the app's two <main>s. Make it a headerless/rail-less
PageShell config (its banner scrolls from the top) so there is exactly
ONE page scroller. Verified: single <main>, banner scrolls, in-page and
browser Back both return to the feed; E2E 11/11.

frontend/src/App.tsx

@@ -689,6 +689,7 @@ export default function App() {
       )}
       <div className="relative flex-1 min-w-0 flex flex-col">
         {channelView ? (
+          <PageShell>
           <Suspense fallback={pageFallback}>
             <ChannelPage
               key={channelView.id}
@@ -714,6 +715,7 @@ export default function App() {
               }}
             />
           </Suspense>
+          </PageShell>
         ) : (
           <PageShell
             header={

frontend/src/components/ChannelPage.tsx

@@ -5,7 +5,6 @@ import { useNavigationActions } from "./NavigationProvider";
 import { ArrowLeft, Ban, Check, ExternalLink, Loader2, Plus, RefreshCw, SlidersHorizontal } from "lucide-react";
 import Avatar from "./Avatar";
 import Feed from "./Feed";
-import PageScroller from "./PageScroller";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -206,8 +205,10 @@ export default function ChannelPage({
       ? "pb-2 text-sm font-medium border-b-2 border-fg text-fg"
       : "pb-2 text-sm text-muted hover:text-fg border-b-2 border-transparent";
 
+  // The channel page owns no scroller of its own: it's a PageShell config (no shared header, no
+  // rail, its banner scrolls from the top) so there's ONE page scroller in the app, not two.
   return (
-    <PageScroller>
+    <>
       {/* Banner + back — OPTION C: inset, rounded card (not full-bleed) */}
       <div className="relative px-4 sm:px-6 pt-3">
         {ch?.banner_url && bannerAttempt <= MAX_BANNER_RETRIES ? (
@@ -466,6 +467,6 @@ export default function ChannelPage({
           )}
         </div>
       )}
-    </PageScroller>
+    </>
   );
 }