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/PlexSearchBox.dom.test.tsx

@@ -180,6 +180,25 @@ describe("PlexSearchBox — typeahead", () => {
     );
   });
 
+  // E11 S4: SearchBar's Escape hands the keyboard back to the page (blur). With the panel open, the
+  // first Escape belongs to the panel: it must close it and leave the caret in the field; only the
+  // next Escape leaves.
+  it("spends the first Escape on the panel and only the second on leaving the field", async () => {
+    plexState.q = "matrix";
+    plexLibrary.mockResolvedValue(result([card("m1", "The Matrix")]));
+    renderBox();
+    const input = screen.getByRole("combobox");
+    input.focus();
+    await screen.findByRole("listbox");
+
+    fireEvent.keyDown(input, { key: "Escape" });
+    await waitFor(() => expect(screen.queryByRole("listbox")).toBeNull());
+    expect(document.activeElement).toBe(input);
+
+    fireEvent.keyDown(input, { key: "Escape" });
+    expect(document.activeElement).not.toBe(input);
+  });
+
   it("marks the input as a combobox and toggles aria-expanded with the panel", async () => {
     plexState.q = "matrix";
     plexLibrary.mockResolvedValue(result([card("m1", "The Matrix")]));