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/plexPlayerHooks.ts

@@ -17,6 +17,7 @@ import {
 } from "./fullscreen";
 import { api, type PlexItemDetail, type PlexMarker } from "./api";
 import { qk } from "./queryKeys";
+import { plexLang } from "../i18n";
 import { type PlexPlayerPrefs } from "./plexPlayerPrefs";
 import { PLEX_SCALE, normalizeVolumeIn, stepVolumeIn } from "./playerVolume";
 
@@ -126,7 +127,10 @@ export function usePlexProgress(opts: {
       /* eslint-disable react-hooks/exhaustive-deps */
       if (absRef.current > 0 && durationRef.current > 0) {
         const pos = Math.floor(absRef.current);
-        qc.setQueryData(qk.plexItem(id), (old) => (old ? { ...old, position_seconds: pos } : old));
+        // Match the language-keyed detail query (E5 S6) so the optimistic resume patch lands.
+        qc.setQueryData(qk.plexItem(id, plexLang()), (old) =>
+          old ? { ...old, position_seconds: pos } : old,
+        );
       }
       /* eslint-enable react-hooks/exhaustive-deps */
       qc.invalidateQueries({ queryKey: qk.plexLibrary() });