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/DownloadCenter.tsx

@@ -17,6 +17,7 @@ import {
 } from "lucide-react";
 import clsx from "clsx";
 import Tabs, { usePersistedTab } from "./Tabs";
+import { PageToolbar } from "./PageShell";
 import Modal from "./Modal";
 // Lazy: these dialogs/editors open on demand, so they stay out of the Downloads page chunk until
 // used (the video editor in particular is heavy — filmstrip + scrubber).
@@ -600,7 +601,10 @@ export default function DownloadCenter({ me }: { me: Me }) {
   );
 
   return (
-    <div className="p-4 max-w-4xl w-full mx-auto">
+    <>
+      {/* Page title + subtitle + tabs are fixed chrome; the tab content scrolls under them. */}
+      <PageToolbar>
+        <div className="px-4 pt-3 max-w-4xl w-full mx-auto">
       <div className="flex items-center justify-between gap-3 mb-1">
         <h1 className="text-xl font-semibold">{t("downloads.page.title")}</h1>
         <button
@@ -613,7 +617,10 @@ export default function DownloadCenter({ me }: { me: Me }) {
       <p className="text-sm text-muted mb-4">{t("downloads.page.subtitle")}</p>
 
       <Tabs tabs={tabs} active={tab} onChange={setTab} />
+        </div>
+      </PageToolbar>
 
+      <div className="px-4 pb-4 pt-3 max-w-4xl w-full mx-auto">
       {tab === "queue" && (
         <>
           <div className="flex gap-2 mb-4">
@@ -739,5 +746,6 @@ export default function DownloadCenter({ me }: { me: Me }) {
         {editJob && <VideoEditor job={editJob} onClose={() => setEditJob(null)} />}
       </Suspense>
     </div>
+    </>
   );
 }