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/features/header/blurActive.ts

@@ -0,0 +1,3 @@
+// A live-filter search box's Go: the list already filters as you type, so Go/Enter only drops the
+// input's focus (the feed's Go is different: it escalates to a YouTube search).
+export const blurActive = () => (document.activeElement as HTMLElement | null)?.blur?.();