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/components/PlexDiscover.dom.test.tsx

@@ -42,6 +42,17 @@ vi.mock("../lib/api", () => ({
     plexSurprise: (...a: unknown[]) => plexSurprise(...a),
     plexSetState: (...a: unknown[]) => plexSetState(...a),
   },
+  // PlexDiscover now reads the active filters via usePlex (PlexProvider); its context default needs
+  // EMPTY_PLEX_FILTERS at import time. No provider is rendered, so the default (empty) filters apply.
+  EMPTY_PLEX_FILTERS: {
+    genres: [],
+    contentRatings: [],
+    directors: [],
+    actors: [],
+    studios: [],
+    keywords: [],
+    collections: [],
+  },
 }));
 
 import PlexDiscover from "./PlexDiscover";
@@ -102,7 +113,8 @@ describe("PlexDiscover", () => {
     expect(screen.getAllByText("Just Added").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Random Pick").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Rec One").length).toBeGreaterThan(0);
-    expect(plexDiscover).toHaveBeenCalledWith("both", "en");
+    // Now called with the active filters too (empty default here, no provider mounted).
+    expect(plexDiscover).toHaveBeenCalledWith("both", "en", expect.objectContaining({ genres: [] }));
   });
 
   it("fires onGenre with the tile's value", async () => {