feat(keys): j/k walk the feed, s finds the search box, g jumps to a module

A feed card cost ~10 Tab stops, so the tenth card sat ~135 Tabs from the top of the page, and the
only keyboard focus ring (a 60% accent outline on the thumbnail) all but vanished on the dark glass.
First slice of the E11 keyboard layer (S4a); Home, Plex and the tables follow in S4b/S4c.

- `lib/listNav`: one window listener over every registered list, walked in document order. `j`/`k`
  step item to item (on into the next list — owner decision D4), the arrows move around a grid, Enter
  clicks the item's own `data-nav-open` element, so it opens exactly like a click. An item root is
  never a Tab stop: it gets `tabindex=-1` only while the keyboard is on it, and keeps it until the next
  item takes it, so the player's focus restore lands back on the card. The first `j` takes the first
  card you can see, not the top of the list.
- `VirtualGrid` takes part with `keyboardNav` (column count + scrolling an unmounted row in);
  `VirtualFeed` turns it on, so the feed, the library and the channel page all have it.
- Keymap: two-step sequences (`g h`) whose pending step belongs to the scope that started it; the
  page-level scopes (`page`, `search`, `list`) are quiet while a layer that owns the keyboard is open,
  enforced in `dispatch`. A test pins the owner's rule that none of them needs a modifier on the
  Hungarian layout.
- `g h/f/c/p/x/n/m/d/s/,` jump to a module this account can reach; `s` focuses the header search box
  and selects the old query; Escape in it hands the keyboard back to the page (Plex's typeahead spends
  its first Escape on its panel).
- Hover popovers are passive layers: an open tooltip still takes Escape first but no longer silences
  the page keys.
- Focus ring (owner decision D2 = B): solid 2px accent around the whole item; the app-wide button/
  link/row ring goes from 60% to solid too. The item keeps a transparent resting outline because the
  video card transitions `all` — without it every `j` flashed a 3px white ring (measured in Chrome).
- Help sheet: the new keys, in a fixed order after the surface's own; a `g` chord only for a reachable
  module; opened over a player or dialog it leaves out the page-level keys and the context-menu key,
  which nothing can reach there (the latter pre-existing since E10).
- CLAUDE.md: a canonical-patterns row for list/grid keys (`useListNav` + `navItemAttrs` +
  `data-nav-open`, not a private arrow-key handler).

frontend/src/components/ui/KeyboardHelp.tsx

@@ -5,9 +5,12 @@ import {
   closeKeyboardHelp,
   formatChord,
   helpLines,
+  isPageLevelScope,
   useActiveScopes,
   useHelpConditionValues,
   useKeyboardHelpOpen,
+  useKeyboardHelpOverLayer,
+  useNavigablePages,
 } from "../../lib/keymap";
 
 // The app's one keyboard cheat sheet. Mounted once by App; the global `h` that opens it lives in
@@ -29,11 +32,21 @@ export default function KeyboardHelp() {
   const open = useKeyboardHelpOpen();
   const scopes = useActiveScopes();
   const conditions = useHelpConditionValues();
+  const overLayer = useKeyboardHelpOverLayer();
+  const pages = useNavigablePages();
 
   if (!open) return null;
   // A `when` row is listed only where it applies — "Previous / next episode" inside a movie would be
   // a promise the player does not keep.
-  const lines = helpLines(scopes).filter((l) => !l.when || !!conditions[l.when]);
+  // Opened over a player or a dialog, the page-level keys (`s`, `g …`, `j`/`k`) are quiet — listing
+  // them there would promise keys that do nothing until the overlay closes. And a `g` chord is listed
+  // only for a module this account can actually reach.
+  // The context-menu key goes too: every `useContextMenu` is a page list, and with an overlay up the
+  // focus Shift+F10 needs is inside the overlay (pre-existing since E10, found in S4 UAT).
+  const live = overLayer
+    ? scopes.filter((s) => !isPageLevelScope(s) && s !== "contextMenu")
+    : scopes;
+  const lines = helpLines(live, pages).filter((l) => !l.when || !!conditions[l.when]);
   return (
     <Modal title={t("keys.title")} onClose={closeKeyboardHelp} maxWidth="max-w-md">
       <dl className="grid grid-cols-[auto_1fr] items-center gap-x-4 gap-y-2.5 text-sm">