feat(plex): S6 frontend — language threading, rating badge, i18n parity

E5 S6 frontend half.

Language threading (?lang=hu serves the HU overlay; en is the default):
- api/plex.ts: `lang` on plexLibrary / plexFacets / plexItem / plexShow (only
  sent for hu, so EN URLs stay byte-stable); PlexItemDetail gains `rating`.
- queryKeys: plexItem / plexShow take an optional trailing lang so a switch caches
  HU/EN separately; the ...rest keys (library/facets) already carry it. Prefix
  invalidations (qk.plexShow(id) / qk.plexItem(id)) still match either.
- i18n: plexLang() collapses the UI language to en|hu for non-component code.
- PlexBrowse / PlexSidebar: thread lang into the grid, facets, item and show
  queries (useTranslation re-renders on switch → the key changes → refetch).
- plexPlayerHooks: the optimistic resume setQueryData now keys by plexLang() so
  it still lands on the language-keyed detail query.
- Clear the genre-filter selection on a language switch — genre chips are
  language-specific strings, so a stale one silently empties the grid.

IMDb-only rating badge (folded-in S5b review #5):
- PlexInfo.tsx shows the IMDb badge + imdb.com link only when imdb_rating is
  present; otherwise it renders the Plex audience `rating` (no IMDb link/branding),
  so a non-IMDb score is never mislabelled. ratingMin filter keys off whichever
  score is shown.

Tests: i18n parity.test.ts (en/hu key sets identical per namespace — currently
green, no drift). FE gate green: tsc / knip / lint clean, 213 vitest tests pass.

frontend/src/components/PlexSidebar.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
+import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Layers, ListMusic, Plus, SlidersHorizontal, X } from "lucide-react";
 import { api, plexFilterCount, type PlexFilters } from "../lib/api";
@@ -80,9 +81,10 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
     onOpenPlaylist(pl.id);
   }
   const { collectionTitle: _ct, ...facetKey } = filters;
+  const lang = plexLang(); // HU/EN genre vocabulary — refetch facets on a language switch
   const facetsQ = useQuery({
-    queryKey: qk.plexFacets(scope, show, facetKey),
-    queryFn: () => api.plexFacets(scope, filters, show),
+    queryKey: qk.plexFacets(scope, show, facetKey, lang),
+    queryFn: () => api.plexFacets(scope, filters, show, lang),
     enabled: !!scope,
     placeholderData: (prev) => prev,
   });