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/plexCards.dom.test.tsx

@@ -5,7 +5,7 @@
 // aria-label — a dropped label silently reintroduces an unnamed button for screen-reader users. i18n is
 // key-passthrough, so the asserted names are the i18n keys.
 import { describe, it, expect, afterEach, vi } from "vitest";
-import { render, screen, cleanup } from "@testing-library/react";
+import { render, screen, cleanup, fireEvent } from "@testing-library/react";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
@@ -100,3 +100,50 @@ describe("EpisodeCard a11y", () => {
     expect(screen.getByRole("button", { name: "plex.playlist.addEpisode" })).toBeTruthy();
   });
 });
+
+// E11 S4b: in a keyboard-walkable grid or rail the WHOLE card is the item (the ring goes round the
+// poster and its title, D2) and the poster — the card's own role=button — is what Enter opens.
+describe("Plex cards in a keyboard-walkable list", () => {
+  const cases = [
+    [
+      "PlexPosterCard",
+      (onOpen: () => void, navIndex?: number) => (
+        <PlexPosterCard
+          card={movie()}
+          onOpen={onOpen}
+          onInfo={vi.fn()}
+          onToggleWatched={vi.fn()}
+          navIndex={navIndex}
+        />
+      ),
+    ],
+    [
+      "SeasonCard",
+      (onOpen: () => void, navIndex?: number) => (
+        <SeasonCard se={season()} onOpen={onOpen} navIndex={navIndex} />
+      ),
+    ],
+    [
+      "EpisodeCard",
+      (onOpen: () => void, navIndex?: number) => (
+        <EpisodeCard ep={episode()} onPlay={onOpen} navIndex={navIndex} />
+      ),
+    ],
+  ] as const;
+
+  it.each(cases)("%s: the root is the item and the poster is its open element", (_, card) => {
+    const onOpen = vi.fn();
+    const { container } = render(card(onOpen, 3));
+    const root = container.firstElementChild as HTMLElement;
+    expect(root.getAttribute("data-nav-index")).toBe("3");
+    const open = root.querySelector<HTMLElement>("[data-nav-open]")!;
+    expect(open.getAttribute("role")).toBe("button");
+    fireEvent.click(open);
+    expect(onOpen).toHaveBeenCalledTimes(1);
+  });
+
+  it.each(cases)("%s: outside a list it is no item", (_, card) => {
+    const { container } = render(card(vi.fn()));
+    expect(container.querySelector("[data-nav-index]")).toBeNull();
+  });
+});