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/components/ui/pageHeaderSlot.ts

@@ -0,0 +1,14 @@
+import { createContext } from "react";
+
+// The floating header's slot (see PageHeader in PageShell): the node a page portals its header
+// controls into, whether a page has claimed it, and how a page claims it. Null with no shell, or in a
+// shell without a header (the channel page), so a page renders its header nowhere there. Its own
+// module so PageShell exports only components (Fast Refresh).
+export type PageHeaderSlotValue = {
+  target: HTMLElement | null;
+  claimed: boolean;
+  /** Claim the header for this page's controls; the returned function releases it. */
+  claim: () => () => void;
+};
+
+export const PageHeaderSlot = createContext<PageHeaderSlotValue | null>(null);