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/i18n/locales/en/keys.json

@@ -21,5 +21,18 @@
   "back": "Stop & back to feed",
   "browseBack": "Back one level",
   "closePlayer": "Close the player",
-  "palette": "Open the jump-to palette"
+  "palette": "Open the jump-to palette",
+  "goFeed": "Go to Feed",
+  "goChannels": "Go to Channels",
+  "goPlaylists": "Go to Playlists",
+  "goPlex": "Go to Plex",
+  "goNotifications": "Go to Notifications",
+  "goMessages": "Go to Messages",
+  "goDownloads": "Go to Downloads",
+  "goStats": "Go to Stats",
+  "goSettings": "Go to Settings",
+  "search": "Jump to the search box",
+  "listStep": "Next / previous item",
+  "gridMove": "Move around the list",
+  "open": "Open the selected item"
 }

frontend/src/i18n/locales/hu/keys.json

@@ -21,5 +21,18 @@
   "back": "Leállítás és vissza a feedre",
   "browseBack": "Vissza egy szintet",
   "closePlayer": "Lejátszó bezárása",
-  "palette": "Ugrás-paletta megnyitása"
+  "palette": "Ugrás-paletta megnyitása",
+  "goFeed": "Ugrás: Hírfolyam",
+  "goChannels": "Ugrás: Csatornák",
+  "goPlaylists": "Ugrás: Lejátszási listák",
+  "goPlex": "Ugrás: Plex",
+  "goNotifications": "Ugrás: Értesítések",
+  "goMessages": "Ugrás: Üzenetek",
+  "goDownloads": "Ugrás: Letöltések",
+  "goStats": "Ugrás: Statisztika",
+  "goSettings": "Ugrás: Beállítások",
+  "search": "Ugrás a keresőbe",
+  "listStep": "Következő / előző elem",
+  "gridMove": "Mozgás a listában",
+  "open": "A kijelölt elem megnyitása"
 }