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/lib/keymap.test.ts

@@ -101,7 +101,6 @@ describe("dispatch", () => {
       mute: noop,
       audio: () => calls.push("audio"),
       subtitle: noop,
-      help: noop,
       info: noop,
       stats: noop,
       back: () => calls.push("back"),
@@ -137,6 +136,20 @@ describe("dispatch", () => {
     input.remove();
   });
 
+  // A `handledLocally` row is documentation + a shared chord, matched on the focused element by its
+  // own component (Shift+F10 belongs to whichever row has focus). `dispatch` must not claim it, and
+  // the mapped handler type must not demand one — this test is the runtime half of that.
+  it("does not route a locally-handled row", () => {
+    const calls: string[] = [];
+    // The handler map is the global scope's routable ids only — `contextMenu` is absent from it,
+    // which is the compile-time half of the same claim; tsc rejects passing one.
+    const handlers = { help: () => calls.push("help") };
+    expect(dispatch(press({ key: "F10", shiftKey: true }), "global", handlers)).toBe(false);
+    expect(dispatch(press({ key: "ContextMenu" }), "global", handlers)).toBe(false);
+    expect(dispatch(press({ key: "h" }), "global", handlers)).toBe(true);
+    expect(calls).toEqual(["help"]);
+  });
+
   it("ignores keys belonging to another scope", () => {
     const calls: string[] = [];
     // `m` (mute) is a plexPlayer key; the browse scope only owns Backspace.
@@ -190,7 +203,7 @@ describe("helpLines", () => {
   it("orders scopes as asked, not as tabled", () => {
     const first = (scopes: Parameters<typeof helpLines>[0]) => helpLines(scopes)[0]?.labelKey;
     expect(first(["plexBrowse", "global"])).toBe("keys.browseBack");
-    expect(first(["global", "plexBrowse"])).toBe("keys.contextMenu");
+    expect(first(["global", "plexBrowse"])).toBe("keys.help");
   });
 });