feat(keys): j/k and the arrows walk Home's rails and every Plex grid and rail

Second slice of the E11 keyboard layer (S4b). S4a gave the feed, the library and the channel page
`j`/`k`, the arrows and Enter; Home's shelves and the whole Plex module were still Tab-stop country —
a Plex poster is its own Tab stop, and a rail's cards sat behind every control of the cards before.

- `lib/listNav`: a list that mounts every item no longer declares a count or columns — both are
  measured off the DOM when a key is pressed, so an auto-fill grid's columns follow its width and a
  rail is one row. ↑ on a list's top row and ↓ on its bottom row go on to the neighbouring list, onto
  the item horizontally nearest the one left (owner decision D4: ←→ within a rail, ↑↓ to the rail
  above or below). The first `j` skips cards a rail has scrolled out to the left.
- `Carousel keyboardNav` registers the scroller; the scroller gets a 4px inset (against a -4px margin,
  layout unchanged) so the focus ring is not clipped, and `[data-nav-index]` a 2.5rem inline scroll
  margin so a card stepped into view clears the rail's edge fade and paging arrow.
- `ui/NavGrid`: the plain, fully mounted grid that registers itself.
- `navIndex` on PlexPosterCard / SeasonCard / EpisodeCard (the card root is the item, the poster the
  open element — the ring goes round poster and title, D2), PosterTile and Home's ShelfTile (the button
  is the item); cast, related and collection-strip buttons carry the attrs directly.
- Wired: Home's comfort and resume rails; Plex Browse titles + episodes, Discover rails + genres,
  collection view, playlists/collections browse, person known-for + filmography, show seasons, season
  episodes, cast/related strips, info-page cast + collection strips.
- `listNav.wiring.contract.test`: every `Carousel` says `keyboardNav`, every card says `navIndex`, and a
  card with `navIndex` sits inside a NavGrid / Carousel / VirtualGrid — both halves are opt-in, and
  either one missing leaves a list the keyboard silently skips.

frontend/src/components/PlexDiscover.dom.test.tsx

@@ -6,7 +6,7 @@
 // payload shows the empty message. i18n is key-passthrough; the api is mocked so queries resolve
 // synchronously. The rails scroll inside a Carousel (useHScroll → ResizeObserver), stubbed for jsdom.
 import { describe, it, expect, afterEach, beforeAll, vi } from "vitest";
-import { render, screen, fireEvent, cleanup, waitFor } from "@testing-library/react";
+import { act, render, screen, fireEvent, cleanup, waitFor } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
 beforeAll(() => {
@@ -263,3 +263,40 @@ describe("PlexDiscover", () => {
     expect(screen.queryByLabelText("plex.markUnwatched")).toBeNull();
   });
 });
+
+// E11 S4b: every rail and the genre grid are lists the keyboard walks (lib/listNav), so `j` goes on
+// from one to the next in page order (owner decision D4).
+describe("PlexDiscover · keyboard", () => {
+  it("registers every rail and the genre grid, and j walks on from rail to rail", async () => {
+    plexDiscover.mockResolvedValue(fullPayload);
+    plexSurprise.mockResolvedValue({ scope: "both", items: [card("sp1", "Random Pick")] });
+    wrap(
+      <PlexDiscover
+        scope="both"
+        onOpenCard={noop}
+        onInfoCard={noop}
+        onGenre={noop}
+        onSeeAllRecent={noop}
+      />,
+    );
+    await screen.findAllByText("Random Pick");
+    // Five rails (continue, recently watched, recently added, because-you-watched, surprise) + genres.
+    const lists = [...document.querySelectorAll("[data-nav-list]")];
+    expect(lists.length).toBe(6);
+    // …and each holds its cards as items: the genre tiles too, not only the posters.
+    expect(lists.every((list) => list.querySelector("[data-nav-index]"))).toBe(true);
+    const step = () => {
+      act(() => {
+        (document.activeElement ?? document.body).dispatchEvent(
+          new KeyboardEvent("keydown", { key: "j", bubbles: true, cancelable: true }),
+        );
+      });
+      return (document.activeElement as HTMLElement).closest("[data-nav-index]")?.textContent;
+    };
+    const first = step();
+    const second = step();
+    expect(first).toBeTruthy();
+    expect(second).toBeTruthy();
+    expect(second).not.toBe(first); // one card per rail: the second press crossed into the next rail
+  });
+});