refactor(frontend): each page fills the floating header itself; the header knows no page

The header used to pick its pills by `page === …` and import five module controls, and Plex's
Discover/Browse toggle needed a context bridge to reach the sub-view that lives in PlexBrowse.
Now the header is an empty band: PageShell hands its root box out as a slot, a page portals its
own pills into it (PageHeader), and the palette's Jump-to pill shows where no page does
(PageHeaderDefault, rendered inside the page's Suspense so a loading page shows no stand-in).
Plex's toggles take the sub-view as a prop; PlexViewBridge and its context are gone, and with
them the header's five layer suppressions.

No visible change: every header pill's box measured identical against the previous build on eight
pages, in Hungarian and English.

frontend/src/modules/playlists/Playlists.tsx

@@ -68,6 +68,7 @@ import { HiddenFilterCue } from "../../components/ui/HiddenFilterCue";
 import { PageHeading } from "../../components/ui/Headings";
 import { useCopyItems } from "../../components/ui/copyItems";
 import RemoteImage from "../../components/ui/RemoteImage";
+import PlaylistsHeader from "./PlaylistsHeader";
 
 // The playlist detail is a HYBRID table: real sortable/filterable column headers over a
 // body that is still a drag-reorderable @dnd-kit list.
@@ -160,7 +161,18 @@ function Row({
   );
 }
 
+// The page: its header pill, and the selected playlist's detail (whose loading and empty states
+// return early, so the header sits outside it).
 export default function Playlists() {
+  return (
+    <>
+      <PlaylistsHeader />
+      <PlaylistDetail />
+    </>
+  );
+}
+
+function PlaylistDetail() {
   // The selected playlist lives in PlaylistsProvider, shared with the PlaylistsRail (which owns the
   // list + its rail controls). This detail pane reacts to the selection.
   const { selectedId } = usePlaylists();