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/queryKeys.ts

@@ -92,9 +92,13 @@ export const qk = {
   plexWatchLink: () => ["plex-watch-link"] as const,
   plexLibrary: (...rest: unknown[]) => ["plex-library", ...rest],
   plexSuggest: (...rest: unknown[]) => ["plex-suggest", ...rest],
-  plexShow: (showId?: string) =>
-    showId !== undefined ? (["plex-show", showId] as const) : (["plex-show"] as const),
-  plexItem: (id: string) => ["plex-item", id] as const,
+  // Optional trailing `lang` (E5 S6): a language switch remounts the detail with a new key so HU/EN
+  // metadata is cached separately. Omitting it (e.g. for a prefix invalidation) still matches both.
+  plexShow: (showId?: string, lang?: string) =>
+    showId !== undefined
+      ? (["plex-show", showId, ...(lang ? [lang] : [])] as const)
+      : (["plex-show"] as const),
+  plexItem: (id: string, lang?: string) => ["plex-item", id, ...(lang ? [lang] : [])] as const,
   plexPlaylists: (...rest: unknown[]) => ["plex-playlists", ...rest],
   plexPlaylist: (playlistId: number) => ["plex-playlist", playlistId] as const,
   plexCollections: (...rest: unknown[]) => ["plex-collections", ...rest],