fix(feed): pin the feed toolbar in the shell's fixed band (bug 4)

The feed's show/content chips, source, count, sort and view switcher —
plus the active-filter chips — lived inside the scroller, so they
scrolled away with the cards; only the SearchBar strip stayed fixed.

Give PageShell a fixed toolbar band (filled by a module via the new
PageToolbar portal) above the scroller. On the feed page the toolbar +
chips portal into it and stay put — and removable — while cards scroll.
The channel-embedded Feed (channelScoped) keeps its inline toolbar,
which rides with the banner/tabs as before.

Verified both entry points at desktop + 1000px (toolbar wraps in-flow,
band grows, still pinned); E2E 11/11; tsc + knip clean.

frontend/src/components/Feed.tsx

@@ -31,6 +31,7 @@ import {
 import { FEED_VIEWS, type FeedView } from "../lib/feedView";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
 import ActiveFilterChips from "./ActiveFilterChips";
+import { PageToolbar } from "./PageShell";
 import ViewSwitcher from "./ViewSwitcher";
 import VirtualFeed from "./VirtualFeed";
 // Lazy: the in-app YouTube player (IFrame API) loads only when a video is first opened.
@@ -654,17 +655,26 @@ export default function Feed({
   );
 
   return (
-    <div className="p-4">
-      {toolbar}
-      {/* Channel pages get no chips: they have no filter panel to reveal (App hides the sidebar for
-          them), and their Feed starts from its OWN baseline — channel-scoped, whole-catalog,
-          show-all — so every one of those would render as a "filter you applied" when it's just the
+    <div className={channelScoped ? "p-4" : "px-4 pb-4 pt-2"}>
+      {/* On the feed page the toolbar + active-filter chips are FIXED chrome: they pin in the
+          shell's toolbar band (above the scroller) so they stay put — and reachable — while the
+          cards scroll (bug 4). On a channel page the Feed is embedded below the channel's own
+          banner/tabs, which scroll, so there the toolbar rides inline with the content as before.
+          Channel pages get no chips: their Feed starts from its OWN baseline (channel-scoped,
+          whole-catalog, show-all), so each would read as a "filter you applied" when it's just the
           page being itself. */}
-      {!channelScoped && (
-        <ActiveFilterChips
-          filters={activeFilters}
-          onClearAll={() => setFilters(clearedFilters(filters))}
-        />
+      {channelScoped ? (
+        toolbar
+      ) : (
+        <PageToolbar>
+          <div className="px-4 pt-4 pb-1">
+            {toolbar}
+            <ActiveFilterChips
+              filters={activeFilters}
+              onClearAll={() => setFilters(clearedFilters(filters))}
+            />
+          </div>
+        </PageToolbar>
       )}
       {items.length === 0 ? (
         <div className="py-16 text-center text-muted">