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/NotificationsPanel.tsx

@@ -21,6 +21,7 @@ import {
 } from "../lib/notifications";
 import { channelYouTubeUrl, relativeFromMs, relativeTime } from "../lib/format";
 import { LEVEL_STYLE } from "./Toaster";
+import { PageToolbar } from "./PageShell";
 import { useNavigationActions } from "./NavigationProvider";
 
 // The single notification center. "System" = durable, server-backed notifications (inbox
@@ -102,8 +103,11 @@ export default function NotificationsPanel({
   }
 
   return (
-    <div className="p-4 max-w-3xl w-full mx-auto space-y-4">
-      <div className="glass rounded-2xl p-4 flex items-center gap-3">
+    <>
+      {/* The inbox header (title + mark-all / clear-all) is fixed chrome; only the list scrolls. */}
+      <PageToolbar>
+        <div className="px-4 pt-3 max-w-3xl w-full mx-auto">
+          <div className="glass rounded-2xl p-4 flex items-center gap-3">
         <Bell className="w-5 h-5 text-accent shrink-0" />
         <div className="flex-1 min-w-0">
           <div className="font-semibold">{t("inbox.title")}</div>
@@ -130,7 +134,10 @@ export default function NotificationsPanel({
           </div>
         )}
       </div>
+        </div>
+      </PageToolbar>
 
+      <div className="px-4 pb-4 pt-3 max-w-3xl w-full mx-auto space-y-4">
       {q.isLoading && !q.data && clientItems.length === 0 ? (
         <div className="p-8 text-muted text-center">{t("common.loading")}</div>
       ) : !hasAny ? (
@@ -185,7 +192,8 @@ export default function NotificationsPanel({
           )}
         </div>
       )}
-    </div>
+      </div>
+    </>
   );
 }