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/PosterTile.tsx

@@ -1,5 +1,6 @@
 import { type ReactNode } from "react";
 import { hueOf, hueTileGradient } from "../lib/hueTile";
+import { navItemAttrs } from "../lib/listNav";
 
 // A browse tile split in two: the TOP half shows up to 2 montage posters (from /api/plex/image/{key}),
 // the BOTTOM half is a solid colour-hash band that carries the label — so the text is always readable
@@ -12,6 +13,7 @@ export default function PosterTile({
   art,
   onClick,
   className,
+  navIndex,
 }: {
   title: string;
   subtitle?: ReactNode; // a bottom sub-line under the title (e.g. "12 items · 4h 30m")
@@ -19,6 +21,7 @@ export default function PosterTile({
   art?: string[]; // up to 2 poster rating_keys; fewer/none falls back to the colour hash
   onClick: () => void;
   className?: string; // caller sets the height (e.g. "h-20" / "h-28")
+  navIndex?: number; // position in a keyboard-walkable grid (lib/listNav); Enter is the button's own
 }) {
   const posters = (art ?? []).slice(0, 2);
   const band = hueTileGradient(hueOf(title));
@@ -27,6 +30,7 @@ export default function PosterTile({
       type="button"
       onClick={onClick}
       title={title}
+      {...(navIndex !== undefined && navItemAttrs(navIndex))}
       className={`glass-hover group relative flex flex-col overflow-hidden rounded-xl text-left transition ${className ?? ""}`}
     >
       {/* Top half: 2 montage posters, or the colour hash when a group has no art. */}