feat(plex): apply the sidebar filters to the Discover rails too

Discover now behaves like the Browse grid — the active metadata filters (genre /
rating / year / collection / studio / …) narrow every rail: Continue watching,
Recently added, Surprise me, the genre tiles, and Because you watched. Same
LibraryFilters / meta_filter_conds the grid uses, applied to each rail's movie +
show query (movie duration included); a rail may come back empty, which the client
already hides. Genre-language matches the grid.

Because Discover honours filters itself, adding a filter no longer force-flips the
Discover landing to the grid (only a header SEARCH does, since Discover has no
search rail) — so you stay on Discover with narrowed rails.

Frontend threads `filters` into api.plexDiscover/plexSurprise + their query keys
(plexSurprise is variadic now) and the watched-toggle setQueryData; PlexDiscover
reads them from usePlex. Tests: two discover filter-narrowing cases; PlexDiscover
dom test updated for the provider default + 3-arg call.

frontend/src/lib/api/plex.ts

@@ -463,16 +463,26 @@ export const plexApi = {
       method: "PUT",
       body: JSON.stringify({ item_rating_keys: itemRks }),
     }),
-  // Discover landing rails in one request (serve-from-DB). `lang` picks the genre-tile vocabulary.
-  plexDiscover: (scope: string, lang?: string): Promise<PlexDiscover> => {
+  // Discover landing rails in one request (serve-from-DB). `lang` picks the genre-tile vocabulary; the
+  // active sidebar `filters` narrow every rail (so Discover behaves like the Browse grid).
+  plexDiscover: (scope: string, lang?: string, filters?: PlexFilters): Promise<PlexDiscover> => {
     const u = new URLSearchParams({ scope });
     if (lang === "hu") u.set("lang", "hu");
+    if (filters) appendPlexFilters(u, filters);
     return req(`/api/plex/discover?${u.toString()}`);
   },
-  // A fresh random sample for the Surprise-me rail — a new draw every call (the ↻ refetch).
-  plexSurprise: (scope: string, limit?: number): Promise<{ scope: string; items: PlexCard[] }> => {
+  // A fresh random sample for the Surprise-me rail — a new draw every call (the ↻ refetch), narrowed by
+  // the active filters like the other rails.
+  plexSurprise: (
+    scope: string,
+    limit?: number,
+    filters?: PlexFilters,
+    lang?: string,
+  ): Promise<{ scope: string; items: PlexCard[] }> => {
     const u = new URLSearchParams({ scope });
     if (limit) u.set("limit", String(limit));
+    if (lang === "hu") u.set("lang", "hu");
+    if (filters) appendPlexFilters(u, filters);
     return req(`/api/plex/surprise?${u.toString()}`);
   },
   plexShow: (id: string, lang?: string): Promise<PlexShowDetail> =>