refactor(shell): introduce PageShell for the normal-page chrome

Collapse App's hand-assembled normal-page chrome (floating Header +
top-padded wrapper + PageScroller + banners) into a single PageShell
contract with declared fixed vs scrollable regions. No behaviour change;
the channel page still renders its own scroller (dissolved next).

Groundwork for the S4 fixed-chrome/scroll-ownership seam.

frontend/src/App.tsx

@@ -53,7 +53,7 @@ import NavSidebar from "./components/NavSidebar";
 import Sidebar from "./components/Sidebar";
 import PlaylistsRail from "./components/PlaylistsRail";
 import BackToTop from "./components/BackToTop";
-import PageScroller from "./components/PageScroller";
+import PageShell from "./components/PageShell";
 import GlassTuner from "./components/GlassTuner";
 import ChatDock from "./components/ChatDock";
 import Toaster from "./components/Toaster";
@@ -715,34 +715,34 @@ export default function App() {
             />
           </Suspense>
         ) : (
-          <>
-        <Header
-          me={meQuery.data!}
-          filters={filters}
-          setFilters={setFilters}
-          plexQ={plexQ}
-          setPlexQ={setPlexQ}
-          channelsSearch={channelsSearch}
-          setChannelsSearch={setChannelsSearch}
-          playlistsSearch={playlistsSearch}
-          setPlaylistsSearch={setPlaylistsSearch}
-          onYtSearch={enterYtSearch}
-          onGoToFullHistory={() => {
-            setChannelFilter("needs_full");
-            setChannelsView("subscribed"); // the status filter applies to the subscriptions tab
-            setChannelsFilterReset((n) => n + 1); // drop any stale column filter hiding the rows
-            setPage("channels");
-          }}
-        />
-        {/* The header floats (fixed) over the content, so it no longer occupies flow space; this
-            wrapper clears it with a top pad. (All left rails, incl. Playlists, are App-level
-            floating panels beside the nav — none sit inside this content column anymore.) */}
-        <div
-          className="flex-1 min-w-0 min-h-0 flex flex-col pt-[var(--hdr-h)]"
-        >
-        {meQuery.data!.is_demo && <DemoBanner />}
-        <VersionBanner onOpen={() => openReleaseNotes(CURRENT_VERSION)} />
-          <PageScroller>
+          <PageShell
+            header={
+              <Header
+                me={meQuery.data!}
+                filters={filters}
+                setFilters={setFilters}
+                plexQ={plexQ}
+                setPlexQ={setPlexQ}
+                channelsSearch={channelsSearch}
+                setChannelsSearch={setChannelsSearch}
+                playlistsSearch={playlistsSearch}
+                setPlaylistsSearch={setPlaylistsSearch}
+                onYtSearch={enterYtSearch}
+                onGoToFullHistory={() => {
+                  setChannelFilter("needs_full");
+                  setChannelsView("subscribed"); // the status filter applies to the subscriptions tab
+                  setChannelsFilterReset((n) => n + 1); // drop any stale column filter hiding the rows
+                  setPage("channels");
+                }}
+              />
+            }
+            banners={
+              <>
+                {meQuery.data!.is_demo && <DemoBanner />}
+                <VersionBanner onOpen={() => openReleaseNotes(CURRENT_VERSION)} />
+              </>
+            }
+          >
           <Suspense fallback={pageFallback}>
           {page === "channels" ? (
             <Channels
@@ -823,9 +823,7 @@ export default function App() {
             />
           )}
           </Suspense>
-          </PageScroller>
-          </div>
-          </>
+          </PageShell>
         )}
         {/* Toasts rise from the bottom-left, by the notification bell in the nav rail.
             Anchored inside the content column so they clear the sidebar automatically

frontend/src/components/PageShell.tsx

@@ -0,0 +1,42 @@
+import { type ReactNode } from "react";
+import PageScroller from "./PageScroller";
+
+// The page shell: the fixed chrome around a single scrollable content region. It formalizes "what
+// is fixed vs what scrolls" per module, so the chrome App used to hand-assemble around every page
+// (a floating Header + a top-padded wrapper + PageScroller, plus the channel page's separate
+// branch that rendered its OWN scroller) collapses to one contract and one scroller.
+//
+// Regions, top to bottom:
+//  - `header`  — floats (absolute, height `var(--hdr-h)`) over the content column; the wrapper pads
+//                its top by that height so nothing hides under it. Omit it (channel page) → no pad
+//                and no fixed top band, so the module's own banner can scroll from the very top.
+//  - `banners` — in-flow, fixed notices (demo/version) above the scroll region.
+//  - `toolbar` — in-flow, fixed per-module band (filter chips / sort / counts) that must NOT scroll
+//                with the content.
+//  - children  — the ONE scrollable region. `PageScroller` owns the scroller and its two
+//                invariants (`scrollbar-gutter: stable`, edge fade); see that file.
+//
+// The shell renders a fragment so it drops straight into App's `relative` content column — the
+// floating header keeps resolving its `absolute` position against that same ancestor.
+export default function PageShell({
+  header,
+  banners,
+  toolbar,
+  children,
+}: {
+  header?: ReactNode;
+  banners?: ReactNode;
+  toolbar?: ReactNode;
+  children: ReactNode;
+}) {
+  return (
+    <>
+      {header}
+      <div className={`flex-1 min-w-0 min-h-0 flex flex-col ${header ? "pt-[var(--hdr-h)]" : ""}`}>
+        {banners}
+        {toolbar}
+        <PageScroller>{children}</PageScroller>
+      </div>
+    </>
+  );
+}