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/settings/SettingsPanel.tsx

@@ -60,8 +60,8 @@ const TABS: { id: TabId; icon: typeof Monitor }[] = [
 ];
 
 // Settings as a page (Design B): rendered in the main content area, not an overlay. It keeps
-// the frosted `.glass` surface so it still refracts the ambient backdrop. The page title is
-// shown by the Header; the tab rail stays as in-page sub-navigation.
+// the frosted `.glass` surface so it still refracts the ambient backdrop. The page's identity is
+// the nav rail and the tab title; the tab rail stays as in-page sub-navigation.
 export default function SettingsPanel() {
   const me = useMe();
   const { t } = useTranslation();