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/plex/PlexScopeToggle.dom.test.tsx

@@ -2,8 +2,8 @@
 //
 // Contract of the header library-scope pill (scopes + browse modes): it renders the
 // three library scopes PLUS the Playlist/Collection browse-mode entries, marks the active one, writes a
-// picked scope to the provider, and drives the playlists/collections views through the PlexViewBridge.
-// i18n is key-passthrough and both the provider and the bridge are mocked; SegmentedToggle is a plain
+// picked scope to the provider, and drives the playlists/collections views through the `view` it is handed.
+// i18n is key-passthrough and the provider is mocked; SegmentedToggle is a plain
 // button group (no floating-ui), so no observer stubs are needed.
 import { describe, it, expect, afterEach, beforeEach, vi } from "vitest";
 import { render, screen, fireEvent, cleanup } from "@testing-library/react";
@@ -20,13 +20,11 @@ vi.mock("./plexContext", () => ({
   usePlexActions: () => ({ setScope }),
 }));
 
-// The bridge: a controllable current view kind + an open spy.
-const controls = { kind: "discover" as string, open: vi.fn() };
-vi.mock("./plexViewContext", () => ({
-  usePlexViewControls: () => controls,
-}));
+// The sub-view: a controllable current view kind + an open spy.
+const controls = { kind: "discover" as PlexViewControls["kind"], open: vi.fn() };
 
 import PlexScopeToggle from "./PlexScopeToggle";
+import type { PlexViewControls } from "./plexSubUrl";
 
 beforeEach(() => {
   plexState.scope = "both";
@@ -48,7 +46,7 @@ const pressed = () =>
 describe("PlexScopeToggle", () => {
   it("renders the three scopes plus the two browse modes, the current scope pressed", () => {
     plexState.scope = "movie";
-    render(<PlexScopeToggle />);
+    render(<PlexScopeToggle view={controls} />);
     expect(labels()).toEqual([
       "plex.filter.scopeOpt.both",
       "plex.filter.scopeOpt.movie",
@@ -60,14 +58,14 @@ describe("PlexScopeToggle", () => {
   });
 
   it("writes a picked scope back to the provider (no view switch when already on a title view)", () => {
-    render(<PlexScopeToggle />);
+    render(<PlexScopeToggle view={controls} />);
     fireEvent.click(screen.getByRole("button", { name: "plex.filter.scopeOpt.show" }));
     expect(setScope).toHaveBeenCalledWith("show");
     expect(controls.open).not.toHaveBeenCalled();
   });
 
   it("opens the playlists view (not a scope change) for the Playlist mode", () => {
-    render(<PlexScopeToggle />);
+    render(<PlexScopeToggle view={controls} />);
     fireEvent.click(screen.getByRole("button", { name: "plex.filter.scopeOpt.playlists" }));
     expect(controls.open).toHaveBeenCalledWith({ kind: "playlists" });
     expect(setScope).not.toHaveBeenCalled();
@@ -75,13 +73,13 @@ describe("PlexScopeToggle", () => {
 
   it("marks the Collection mode active when the current view is collections", () => {
     controls.kind = "collections";
-    render(<PlexScopeToggle />);
+    render(<PlexScopeToggle view={controls} />);
     expect(pressed()).toEqual(["plex.filter.scopeOpt.collections"]);
   });
 
   it("returns to the title grid when a scope is picked from a browse mode", () => {
     controls.kind = "playlists";
-    render(<PlexScopeToggle />);
+    render(<PlexScopeToggle view={controls} />);
     fireEvent.click(screen.getByRole("button", { name: "plex.filter.scopeOpt.movie" }));
     expect(setScope).toHaveBeenCalledWith("movie");
     expect(controls.open).toHaveBeenCalledWith({ kind: "grid" });