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>
+    </>
   );
 }

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>
+    </>
   );
 }

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>
+    </>
   );
 }