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/PlayerModal.tsx

@@ -44,7 +44,7 @@ import {
 } from "../lib/format";
 import { renderDescription } from "../lib/descriptionLinks";
 import { fullscreenElement } from "../lib/fullscreen";
-import { dispatch, isActivationTarget } from "../lib/keymap";
+import { dispatch, isActivationTarget, useKeyScope } from "../lib/keymap";
 import { useBackToClose } from "../lib/history";
 import {
   useNativeControlsYield,
@@ -330,6 +330,8 @@ export default function PlayerModal({
     };
   }, []);
 
+  useKeyScope("ytPlayer");
+
   // Keyboard shortcuts + background scroll lock. The chords live in the shared keymap (`ytPlayer`
   // scope) so the help sheet lists them; only the behaviour behind each id is here.
   // These fire only while focus is on our page, not inside the cross-origin player iframe — so