feat(keys): the conversation list walks with j/k; the cheat sheet names keys in the reader's language

E11 S5 — the HU/EN sweep and the keyboard-only pass over every module.

- The keyboard cheat sheet printed "Space" and "Menu" on a Hungarian sheet: a keycap that carries a
  WORD (not a legend every layout prints alike, like Esc, Ctrl or Enter) is now named by the caller in
  the reader's language (`formatChord(chord, words)`, `keys.cap.*`: Szóköz / Menü).
- The Plex player's ⌫/Esc line said "Stop & back to feed", but the player goes back to wherever it
  was opened from (Home, a Plex page): "Stop & go back" / "Leállítás és vissza".
- Messages: the conversation list is a keyboard list (NavGrid + navItemAttrs, the name button is the
  open element; the pop-out button keeps its own Enter). D1 covers every list whose rows open one
  thing; this one was missed in S4. Notification cards have no single open action and stay out, like
  the download queue.

Sweep, measured: every static `t("…")` key used in the source exists in both languages (the only
misses are template prefixes); the HU values equal to EN are proper nouns and loanwords (Admin, Plex,
Trakt, Shorts). Palette in Hungarian: "téma" makes "Téma: Midnight" the active row ahead of the channel
"Topik - mindig témában", live region "7 találat".

Keyboard-only pass in Chrome, Hungarian UI — 1024×768: every `g` chord lands on its module, `j` rings
the first item (solid 2px accent) on Home, Feed, Channels, Playlists, Plex and Downloads, `s` focuses
the search on Feed, Channels, Playlists and Plex, no page scrolls sideways. 3440×1440: `j` → → ↓
walks Home, Feed (8 columns: 0 → 1 → 9), Channels, Playlists, Plex and Downloads, no sideways scroll.
The help sheet lists every line translated; inside the Plex player "Szóköz / K" and "⌫ / Esc
Leállítás és vissza". Messages: a real `j` rings the row, Enter opens the thread.

frontend/src/components/PlexPlayer.tsx

@@ -915,7 +915,8 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
         info: () => setInfoOpen((v) => !v),
         stats: () => setStatsOpen((v) => !v),
         back: (ev) => {
-          // HTPC-style "stop & back to the feed" — mirrors the mouse Back button (shared cascade).
+          // HTPC-style "stop & go back" to wherever the player was opened from — mirrors the mouse
+          // Back button (shared cascade).
           // Who owns ⌫ when something is open over the player. Modal stopPropagation()s Escape ONLY
           // and its focused close button is not a typing target, so the key reaches us regardless.
           // Two wrong answers so far: deferring to nothing at all (round 1's bug — the cheat sheet