fix(player): a related video played is tracked, and its list is honest about its state

A related video played in place is the player's item now: its title and its own watched /
playlist / download controls, its progress checkpointed and resumed, marked watched at its end —
and its end does not step the queue. Back returns to the opened video, where it was left.

The related list:
- its marks announce through the shared notice (the Notification Center finds them), join the
  host's player session (the feed refetches on close), and stay per write, so an earlier answer no
  longer undoes a later mark still on its way;
- every host's state write patches the cached related lists, which are never refetched;
- kept for the session (gcTime), a retry asks for the page that failed, a list read so far with
  nothing unseen says so, a refusal a retry cannot answer offers none, and the followed mark names
  itself on hover.

Turned off on the instance (a limit of 0), the tab, the drawer button and the live-search buttons
are left out.

Keys: the player's keys and its focus reclaim defer to a layer above it (the drawer); going wide
moves the drawer's list into the panel's tab; `j` goes on from the slot of a row removed inside the
top layer; the help sheet lists the list keys over a layer that holds a list.

frontend/src/components/ui/KeyboardHelp.dom.test.tsx

@@ -16,7 +16,8 @@ import {
 } from "../../lib/keymap";
 import { pushLayer, removeLayer } from "../../lib/layerStack";
 import { useMediaKeys } from "../../lib/mediaKeys";
-import { useState } from "react";
+import { useRef, useState } from "react";
+import { navItemAttrs, useListNav } from "../../lib/listNav";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
@@ -144,6 +145,40 @@ describe("KeyboardHelp", () => {
     expect(descriptions()).toContain("keys.help");
   });
 
+  // The player's related videos are a list inside the layer: their keys work there (`inTopLayer`), so
+  // the sheet opened over the player lists them — and over a layer with no list, still does not.
+  it("lists the list and menu keys over a layer that holds a list of its own", () => {
+    function LayerList() {
+      const ref = useRef<HTMLDivElement>(null);
+      useListNav(ref, { enabled: true });
+      useContextMenu<string>();
+      return (
+        <div data-testid="layer">
+          <div ref={ref}>
+            <div {...navItemAttrs(0)}>row</div>
+          </div>
+        </div>
+      );
+    }
+    render(
+      <Host>
+        <LayerList />
+      </Host>,
+    );
+    const withList = pushLayer({ root: () => screen.getByTestId("layer") });
+    pressH();
+    expect(descriptions()).toContain("keys.listStep");
+    expect(descriptions()).toContain("keys.contextMenu");
+    act(() => closeKeyboardHelp());
+    removeLayer(withList);
+
+    const bare = pushLayer();
+    pressH();
+    removeLayer(bare);
+    expect(descriptions()).not.toContain("keys.listStep");
+    expect(descriptions()).not.toContain("keys.contextMenu");
+  });
+
   // Two live context menus (the Playlists page mounts one, its rail another) must not double the row.
   it("lists the context-menu chords once however many surfaces register", () => {
     function WithMenu() {