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/index.css

@@ -84,7 +84,8 @@
   [role="button"]:focus-visible,
   [role="tab"]:focus-visible,
   summary:focus-visible {
-    outline: 2px solid color-mix(in srgb, var(--accent) 60%, transparent);
+    /* Solid accent (E11 D2 = B): the 60% mix of R9 all but vanished on the dark glass. */
+    outline: 2px solid var(--accent);
     outline-offset: 2px;
   }
 }
@@ -435,10 +436,29 @@ body {
   .selectable:focus-visible {
     /* outline (not a box-shadow ring) so it never fights a glass surface's box-shadow and never gets
        clipped by an overflow-hidden ancestor; inset offset keeps it inside tightly-packed rows. */
-    outline: 2px solid color-mix(in srgb, var(--accent) 60%, transparent);
+    outline: 2px solid var(--accent);
     outline-offset: -2px;
   }
 
+  /* E11 S4 · a list/grid item the keyboard landed on (`lib/listNav`): the SAME solid ring, around the
+     whole card rather than one control inside it (D2 = B). It is focus-visible only, so a mouse click
+     never paints it, and it is a ring rather than a fill, so it never reads like the hover tint. The
+     scroll margins are what `scrollIntoView({block:"nearest"})` honours: they keep a card stepped to
+     from sliding under the floating header (`--hdr-h`) or flush against the bottom edge. */
+  [data-nav-index] {
+    scroll-margin-top: calc(var(--hdr-h, 0px) + 0.75rem);
+    scroll-margin-bottom: 0.75rem;
+    /* A transparent resting ring of the SAME geometry: the video card transitions `all`, so without
+       it the ring animated in from the initial outline (3px, currentColor) — a white flash on every
+       `j`, measured in Chrome. Now only the colour fades in. */
+    outline: 2px solid transparent;
+    outline-offset: 2px;
+  }
+  [data-nav-index]:focus-visible {
+    outline: 2px solid var(--accent);
+    outline-offset: 2px;
+  }
+
   /* ===== R10 S4 · Selection-paint recipes =====
      The ~14 hand-rolled "this is the selected / current / active one" idioms all painted the SAME
      bg-accent fill via an inline ternary (`sel ? "bg-accent text-accent-fg border-accent" : "…"`),