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={