feat(shell): fix headers on Notifications, Downloads, Discovery

Apply the fixed-chrome pattern to three more list modules:
- Notifications: the inbox header (title + mark-all / clear-all) pins in
  the band; only the notification list scrolls.
- Downloads: the title + subtitle + tabs pin; the tab content scrolls.
- Channel discovery: the intro + table controls pin (controlsInBand), the
  table rows scroll under the sticky header (fills out the manager's
  Discover tab to match Subscriptions).

Verified: headers land in the fixed band, content in the scroller. Suite
15/15, tsc + knip clean.

frontend/src/components/ChannelDiscovery.tsx

@@ -10,6 +10,7 @@ import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import Tooltip from "./Tooltip";
 import ChannelLink from "./ChannelLink";
 import DataTable, { type Column } from "./DataTable";
+import { PageToolbar } from "./PageShell";
 import { useConfirm } from "./ConfirmProvider";
 
 // The Channel manager's "Discovery" tab: channels that turn up in the user's playlists but
@@ -154,22 +155,29 @@ export default function ChannelDiscovery({
   ];
 
   return (
-    <div className="px-4 pb-4 pt-3 max-w-7xl mx-auto">
-      <p className="text-xs text-muted mb-4 leading-relaxed">
-        {t("channels.discovery.intro")}
-      </p>
-      {query.isLoading ? (
-        <div className="text-muted py-8">{t("channels.discovery.loading")}</div>
-      ) : (
-        <DataTable
-          rows={rows}
-          columns={columns}
-          rowKey={(c) => c.id}
-          persistKey={accountKey(LS.channelDiscoveryTable) ?? undefined}
-          controlsPosition="top"
-          emptyText={t("channels.discovery.empty")}
-        />
-      )}
-    </div>
+    <>
+      {/* Intro + the table controls are fixed chrome; only the table rows scroll (sticky header). */}
+      <PageToolbar>
+        <div className="px-4 pt-3 max-w-7xl mx-auto">
+          <p className="text-xs text-muted leading-relaxed">{t("channels.discovery.intro")}</p>
+        </div>
+      </PageToolbar>
+      <div className="px-4 pb-4 max-w-7xl mx-auto">
+        {query.isLoading ? (
+          <div className="text-muted py-8">{t("channels.discovery.loading")}</div>
+        ) : (
+          <DataTable
+            rows={rows}
+            columns={columns}
+            rowKey={(c) => c.id}
+            persistKey={accountKey(LS.channelDiscoveryTable) ?? undefined}
+            controlsPosition="top"
+            controlsInBand
+            controlsBandClassName="px-4 max-w-7xl mx-auto"
+            emptyText={t("channels.discovery.empty")}
+          />
+        )}
+      </div>
+    </>
   );
 }