feat(e10): one keyboard cheat sheet for the whole app

The Plex player had a shortcuts overlay nobody else did, so the YouTube
player's Space/F/arrows and the Plex browser's Backspace were documented
nowhere. ui/KeyboardHelp is that sheet for every surface, opened by a
global `h` - not the conventional `?`, which is a Shift chord on a
Hungarian layout, while `h` is the key this app's reader already has in
their fingers from the player.

It renders the shared table filtered to the scopes that are LIVE: a
surface registers its scope with useKeyScope while mounted, and only
while its keys really do something - PlexBrowse deactivates its own
scope inside the player, where its Backspace no-ops, so the sheet cannot
promise a "Back one level" the handler declines to perform. Same rule
for the `when` rows: ⇧←/⇧→ are listed only inside a series, an answer
the player publishes because the sheet cannot ask.

No new overlay primitive, after checking: Modal already portals into
document.fullscreenElement ?? document.body (so this works over a
fullscreen player), registers an Escape layer, traps focus and handles
browser Back. Third time this epic that the thing was already built.

Shift+F10 is in the table as `handledLocally` - documented and sharing
the chord parser, but matched by useContextMenu on the focused row.
dispatch neither routes it nor demands a handler for it, so the
exhaustiveness check stays honest about what it actually covers.

Also: A/S have cycled the Plex audio and subtitle tracks since E5, but
silently - the only way to see where you landed was to open the tracks
menu, the exact round-trip the shortcut exists to avoid. cycleAudio and
cycleSubtitle now return the track they moved to, and the player flashes
it over the video.

frontend/src/components/PlexBrowse.tsx

@@ -39,7 +39,7 @@ import { usePlex, usePlexActions } from "./PlexProvider";
 import { usePublishPlexView } from "./PlexViewBridge";
 import { EpisodeCard, PlexPosterCard } from "./plexCards";
 import { PlexInfoView, PlexPersonView, PlexSeasonView, PlexShowView } from "./plexViews";
-import { dispatch } from "../lib/keymap";
+import { dispatch, useKeyScope } from "../lib/keymap";
 
 // Lazy: the rich player (pulls in hls.js) loads only when something is first played.
 const PlexPlayer = lazy(() => import("./PlexPlayer"));
@@ -65,6 +65,20 @@ const LASTWATCHED_SORT: NonNullable<SortState> = Object.freeze({
 // I looking at" control now, like the feed's Watch-state pill, not a rail widget).
 const SHOW_OPTS = ["all", "unwatched", "in_progress", "watched"] as const;
 
+// The drill-down views Backspace can step out of. "player" is deliberately absent: the player owns
+// its own Backspace (stop & back to the grid), and this handler must not fire underneath it.
+const BACKABLE_VIEWS: readonly string[] = [
+  "grid",
+  "playlists",
+  "collections",
+  "show",
+  "season",
+  "info",
+  "person",
+  "playlist",
+  "collection",
+];
+
 export default function PlexBrowse() {
   const { t } = useTranslation();
   const qc = useQueryClient();
@@ -164,27 +178,16 @@ export default function PlexBrowse() {
   }, [sub.view.kind]);
 
   // Backspace steps back one drill-down level (grid ← show ← season, and out of info/playlist),
-  // matching the browser/mouse Back. The player has its OWN Backspace handling, so we skip it here
-  // (its "player" view kind is deliberately absent from the list below); the typing guard and the
-  // chord itself come from the shared keymap.
+  // matching the browser/mouse Back; the chord and the typing guard come from the shared keymap.
+  // BACKABLE_VIEWS is the ONE list behind both the guard and the scope registration, so the cheat
+  // sheet can never promise a "Back one level" the handler declines to perform.
+  const canStepBack = BACKABLE_VIEWS.includes(sub.view.kind);
+  useKeyScope("plexBrowse", canStepBack);
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
       dispatch(e, "plexBrowse", {
         back: (ev) => {
-          if (
-            ![
-              "grid",
-              "playlists",
-              "collections",
-              "show",
-              "season",
-              "info",
-              "person",
-              "playlist",
-              "collection",
-            ].includes(sub.view.kind)
-          )
-            return;
+          if (!canStepBack) return;
           ev.preventDefault();
           sub.back();
         },
@@ -192,7 +195,7 @@ export default function PlexBrowse() {
     };
     window.addEventListener("keydown", onKey);
     return () => window.removeEventListener("keydown", onKey);
-  }, [sub]);
+  }, [sub, canStepBack]);
 
   // D3: entering the in-progress watch-state view auto-applies the "last watched" sort (most recently
   // watched first); leaving restores the sort that was active before entering. Only the ENTER edge