fix(plex): S6 — thread lang into the player's item_detail query

Found by the HU-overlay E2E: PlexPlayer fetched qk.plexItem(id) WITHOUT lang,
so the in-player title/summary/rich detail stayed English even in HU mode while
the grid showed Hungarian (e.g. the player header read "127 Hours" over a grid
of "127 óra"). The S6 FE pass threaded lang through the grid, facets, info-page
and show queries but missed the player's own detail fetch. Now keyed by
plexLang() like the rest, so the player matches the active language.

FE gate: tsc / knip / lint clean; queryKeys tests pass. Verified live — the
player header now reads "127 óra" in HU mode.

frontend/src/components/PlexPlayer.tsx

@@ -2,6 +2,7 @@ import { Fragment, lazy, Suspense, useCallback, useEffect, useRef, useState } fr
 import { qk } from "../lib/queryKeys";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
+import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Hls from "hls.js";
 import {
@@ -77,7 +78,12 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   tRef.current = t;
   const qc = useQueryClient();
   const [id, setId] = useState(itemId);
-  const detailQ = useQuery({ queryKey: qk.plexItem(id), queryFn: () => api.plexItem(id) });
+  // Serve the player's title/summary/rich detail in the active language (E5 S6), matching the grid.
+  const lang = plexLang();
+  const detailQ = useQuery({
+    queryKey: qk.plexItem(id, lang),
+    queryFn: () => api.plexItem(id, lang),
+  });
   const detail = detailQ.data;
   const duration = detail?.duration_seconds ?? 0;