fix(ui): a toast host ranks by when its overlay opened, and an overlay keeps its Escape, its dialog and its navigation straight

Deep-review round 4, the overlays.

- Toast hosts were ranked by mount time, and PlayerModal swaps its host on every fullscreen change:
  leaving true fullscreen with the download dialog open put the player's host back on top, so toasts
  were drawn under that dialog's scrim and a click on one closed the dialog. Each overlay takes a
  rank once, when it opens (`useOverlayOrder`), and hands it to every host it renders.
- A dialog open while fullscreen is left keeps a stable portal container (moved, not re-created):
  React tore the portal down when `fsRoot` changed, resetting a half-filled form and dropping focus
  to <body>. Focus is put back after the move.
- The onboarding wizard is a layer, not a raw document listener: one Escape used to dismiss the
  error dialog over it AND end onboarding, skipping the write-scope step.
- `navigateTo` (a toast's "View", clicked inside the player's own host) goes over the overlays it
  unmounts, so the player's Back-to-close cleanup no longer pops the page it just opened — the same
  fix Shift+W already had. The access-requests toast action too.
- The live region arms itself a task after it takes over inherited toasts: React flushes effects in
  the same task as that render, so the "silent inheritance" arming was a no-op.
- The overlay-host guard matches class tokens in any order (PlexPlayer's root was never checked),
  globs `ui/` too (ui/Modal was not checked either), and requires the rank.

frontend/src/components/ui/Modal.fullscreen.dom.test.tsx

@@ -0,0 +1,52 @@
+// @vitest-environment jsdom
+//
+// A dialog opened from a player in true fullscreen portals INTO the fullscreen element (a node under
+// <body> is painted beneath it, i.e. invisible). Leaving fullscreen changes that container — and a
+// React portal whose container changes is torn down and recreated, so everything the dialog held went
+// with it: a half-filled download form reset to its defaults and focus fell to <body> (a verifier's
+// side finding in deep-review round 4).
+import { describe, it, expect, afterEach } from "vitest";
+import { render, cleanup, screen, fireEvent, act } from "@testing-library/react";
+import { vi } from "vitest";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+
+import Modal from "./Modal";
+
+const setFullscreen = (el: Element | null) =>
+  act(() => {
+    Object.defineProperty(document, "fullscreenElement", { value: el, configurable: true });
+    document.dispatchEvent(new Event("fullscreenchange"));
+  });
+
+afterEach(() => {
+  cleanup();
+  setFullscreen(null);
+});
+
+describe("a dialog open while fullscreen is left", () => {
+  it("keeps what the viewer had typed in it, and the focus inside it", async () => {
+    const stage = document.createElement("div");
+    document.body.appendChild(stage);
+    await setFullscreen(stage);
+    render(
+      <Modal title="Download" onClose={() => {}}>
+        <input aria-label="name" defaultValue="" />
+      </Modal>,
+    );
+    const field = screen.getByLabelText("name") as HTMLInputElement;
+    fireEvent.change(field, { target: { value: "my cut" } });
+    field.focus();
+    expect(stage.contains(field)).toBe(true);
+
+    await setFullscreen(null); // the browser's own Esc leaves fullscreen; the dialog stays open
+
+    const after = screen.getByLabelText("name") as HTMLInputElement;
+    expect(after.value).toBe("my cut");
+    expect(document.activeElement).toBe(after);
+    stage.remove();
+  });
+});