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/channels/Channels.tsx

@@ -64,6 +64,7 @@ import { useFeedFilters, useFeedFiltersActions } from "../feed";
 import { useWizard } from "../../features/wizard/wizardContext";
 import { useMe } from "../../lib/useMe";
 import { useTags } from "../../lib/useTags";
+import ChannelsHeader from "./ChannelsHeader";
 
 export type ChannelsView = "subscribed" | "discovery" | "hidden" | "blocked";
 
@@ -687,6 +688,7 @@ export default function Channels() {
 
   return (
     <>
+      <ChannelsHeader />
       {pageMenuItems.length > 0 && (
         <PageMenu ariaLabel={t("common.pageMenu")} items={pageMenuItems} />
       )}