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/PlexBrowse.tsx

@@ -42,6 +42,7 @@ import { PlexPinMenu } from "./ComfortPinMenu";
 import { useContextMenu } from "./ui/useContextMenu";
 import { PlexInfoView, PlexPersonView, PlexSeasonView, PlexShowView } from "./plexViews";
 import { dispatch, useKeyScope } from "../lib/keymap";
+import NavGrid from "./ui/NavGrid";
 
 // Lazy: the rich player (pulls in hls.js) loads only when something is first played.
 const PlexPlayer = lazy(() => import("./PlexPlayer"));
@@ -607,10 +608,11 @@ export default function PlexBrowse() {
               {episodes.length > 0 && (
                 <h2 className="text-sm font-semibold mb-2">{t("plex.unified.titles")}</h2>
               )}
-              <div className="grid grid-cols-[repeat(auto-fill,minmax(150px,1fr))] gap-3">
-                {items.map((c) => (
+              <NavGrid className="grid grid-cols-[repeat(auto-fill,minmax(150px,1fr))] gap-3">
+                {items.map((c, i) => (
                   <PlexPosterCard
                     key={c.id}
+                    navIndex={i}
                     card={c}
                     onOpen={() => onCard(c)}
                     onInfo={() => onInfo(c)}
@@ -618,24 +620,25 @@ export default function PlexBrowse() {
                     menuProps={pinMenu.triggerProps}
                   />
                 ))}
-              </div>
+              </NavGrid>
             </>
           )}
           {/* Episodes section (Proposal 3) — matching episodes on a search, kept out of the title grid. */}
           {episodes.length > 0 && (
             <section className="mt-8">
               <h2 className="text-sm font-semibold mb-3">{t("plex.unified.episodes")}</h2>
-              <div className="grid grid-cols-[repeat(auto-fill,minmax(230px,1fr))] gap-x-3 gap-y-4">
-                {episodes.map((ep) => (
+              <NavGrid className="grid grid-cols-[repeat(auto-fill,minmax(230px,1fr))] gap-x-3 gap-y-4">
+                {episodes.map((ep, i) => (
                   <EpisodeCard
                     key={ep.id}
+                    navIndex={i}
                     ep={ep}
                     withShowTitle
                     onPlay={() => onCard(ep)}
                     onToggleWatched={() => toggleWatched(ep)}
                   />
                 ))}
-              </div>
+              </NavGrid>
             </section>
           )}
         </>