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/Home.dom.test.tsx

@@ -671,3 +671,47 @@ describe("Home", () => {
     );
   });
 });
+
+// E11 S4b, owner decision D4: the rails are one keyboard page — `j` walks a rail's cards and goes on
+// into the next rail, whichever order the page stacks them in.
+describe("Home · keyboard", () => {
+  it("walks the rails with j, from one rail on into the next", async () => {
+    discover.mockResolvedValue({
+      ...payload("v1"),
+      continue_watching: [
+        ytCard("v1", "Hero v1"),
+        ytCard("v2", "Second thing"),
+        ytCard("v3", "Third thing"),
+      ],
+      // A pinned channel has no card of its own: the shelf draws it as a ShelfTile.
+      comfort: [
+        ytCard("v4", "Comfort thing"),
+        {
+          ...ytCard("UC9", "Pinned channel"),
+          kind: "channel",
+          video: null,
+          next: null,
+          unwatched_count: 3,
+        },
+      ],
+    });
+    mount();
+    await screen.findByText("Comfort thing");
+    const titles = ["Comfort thing", "Pinned channel", "Second thing", "Third thing"];
+    const seen: (string | undefined)[] = [];
+    for (let i = 0; i < titles.length; i++) {
+      act(() => {
+        (document.activeElement ?? document.body).dispatchEvent(
+          new KeyboardEvent("keydown", { key: "j", bubbles: true, cancelable: true }),
+        );
+      });
+      const item = (document.activeElement as HTMLElement).closest("[data-nav-index]");
+      seen.push(titles.find((title) => item?.textContent?.includes(title)));
+    }
+    // Every card once, across both rails: the shelf tile, and on from one rail into the next.
+    expect(new Set(seen)).toEqual(new Set(titles));
+    // Every card of every rail is an item of it — the shelf tile too, not only the native cards.
+    const rails = [...document.querySelectorAll("[data-nav-list]")];
+    expect(rails.map((rail) => rail.querySelectorAll("[data-nav-index]").length)).toEqual([2, 2]);
+  });
+});