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/lib/api/plex.ts

@@ -121,6 +121,7 @@ export interface PlexItemDetail {
   imdb_rating?: number | null;
   imdb_id?: string | null;
   imdb_url?: string | null;
+  rating?: number | null; // Plex audience score — shown when there's no IMDb score
   content_rating?: string | null;
   genres: string[];
   directors: string[];
@@ -285,6 +286,7 @@ export const plexApi = {
     field?: PlexSearchField; // search scope-pill (all | title | people | summary | episode)
     sort?: SortState;
     show?: string;
+    lang?: string; // active UI language: "hu" serves the HU overlay twin, else EN (default)
     offset?: number;
     limit?: number;
     filters?: PlexFilters;
@@ -297,16 +299,18 @@ export const plexApi = {
       u.set("sort_dir", p.sort.dir);
     }
     if (p.show && p.show !== "all") u.set("show", p.show);
+    if (p.lang === "hu") u.set("lang", "hu"); // en is the server default; omit to keep URLs stable
     if (p.offset) u.set("offset", String(p.offset));
     if (p.limit) u.set("limit", String(p.limit));
     if (p.filters) appendPlexFilters(u, p.filters);
     return req(`/api/plex/library?${u.toString()}`);
   },
   // Facets ADAPT to the active filters + watch-state (faceted search) — pass them so each group
-  // narrows the others and the bounds match the visible result set.
-  plexFacets: (scope: string, f?: PlexFilters, show?: string): Promise<PlexFacets> => {
+  // narrows the others and the bounds match the visible result set. `lang` picks the genre vocabulary.
+  plexFacets: (scope: string, f?: PlexFilters, show?: string, lang?: string): Promise<PlexFacets> => {
     const u = new URLSearchParams({ scope });
     if (show && show !== "all") u.set("show", show);
+    if (lang === "hu") u.set("lang", "hu");
     if (f) appendPlexFilters(u, f);
     return req(`/api/plex/facets?${u.toString()}`);
   },
@@ -400,8 +404,8 @@ export const plexApi = {
       method: "PUT",
       body: JSON.stringify({ item_rating_keys: itemRks }),
     }),
-  plexShow: (id: string): Promise<PlexShowDetail> =>
-    req(`/api/plex/show/${encodeURIComponent(id)}`),
+  plexShow: (id: string, lang?: string): Promise<PlexShowDetail> =>
+    req(`/api/plex/show/${encodeURIComponent(id)}${lang === "hu" ? "?lang=hu" : ""}`),
   // Mark a whole show / season watched or unwatched (all its episodes) for the current user.
   plexShowState: (rk: string, watched: boolean): Promise<{ changed: number; watched: boolean }> =>
     req(`/api/plex/show/${encodeURIComponent(rk)}/state`, {
@@ -413,8 +417,8 @@ export const plexApi = {
       method: "POST",
       body: JSON.stringify({ watched }),
     }),
-  plexItem: (id: string): Promise<PlexItemDetail> =>
-    req(`/api/plex/item/${encodeURIComponent(id)}`),
+  plexItem: (id: string, lang?: string): Promise<PlexItemDetail> =>
+    req(`/api/plex/item/${encodeURIComponent(id)}${lang === "hu" ? "?lang=hu" : ""}`),
   // Cards for the actor filter-chips currently applied: name + cached headshot + in-scope film/show count.
   plexPeopleCards: (names: string[], scope: string): Promise<{ people: PlexPersonCard[] }> =>
     req(