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/App.tsx

@@ -34,6 +34,7 @@ import PageShell from "./components/ui/PageShell";
 import GlassTuner from "./components/GlassTuner";
 import ChatDock from "./components/ChatDock";
 import Toaster from "./components/ui/Toaster";
+import KeyboardHelp from "./components/ui/KeyboardHelp";
 import { StateMessage } from "./components/ui/QueryState";
 import ErrorDialog from "./components/ErrorDialog";
 import VersionBanner from "./components/VersionBanner";
@@ -412,6 +413,9 @@ export default function App() {
             (collapsed or expanded) without tracking its width. */}
         <Toaster />
       </div>
+      {/* The app-wide keyboard cheat sheet + the global `h` that opens it. Mounted once, outside
+          the page area, so every surface answers "what can I press here" the same way. */}
+      <KeyboardHelp />
       {meQuery.data && !meQuery.data.is_demo && <ChatDock meId={meQuery.data.id} />}
       <Suspense fallback={null}>
         {aboutOpen && (