fix(plex): round-3 deep-review follow-ups

Third review round, scoped to the round-2 fix commit. The production ternary was
confirmed correct; two sibling-surface findings:

- The sidebar CollectionPicker's clear-all still wrote collectionTitles:{} while the
  chip-row remove (fixed in round 2) wrote undefined — a divergence that leaves the
  {}-vs-absent cache split on the picker path. Collapse an empty map to undefined there
  too, so both no-collection routes key-match the pristine state.
- The queryKeys lang test was near-tautological (a variadic-spread factory can't drop a
  passed arg), and nothing asserted the CALL SITE keys carry lang. Add a real guard in
  PlexDiscover.dom.test.tsx that inspects the registered query keys for scope+lang, and
  reword the factory test's comment to say it only shape-pins the segment pass-through.

Gate: tsc/lint/knip/prettier clean, 413 vitest.

frontend/src/components/PlexDiscover.dom.test.tsx

@@ -114,7 +114,48 @@ describe("PlexDiscover", () => {
     expect(screen.getAllByText("Random Pick").length).toBeGreaterThan(0);
     expect(screen.getAllByText("Rec One").length).toBeGreaterThan(0);
     // Now called with the active filters too (empty default here, no provider mounted).
-    expect(plexDiscover).toHaveBeenCalledWith("both", "en", expect.objectContaining({ genres: [] }));
+    expect(plexDiscover).toHaveBeenCalledWith(
+      "both",
+      "en",
+      expect.objectContaining({ genres: [] }),
+    );
+  });
+
+  it("registers the discover + surprise queries keyed by the active lang", async () => {
+    // The real cache-separation guard (the queryKeys factory test only pins that a lang ARG survives the
+    // spread): inspect the actual keys the component registered. A regression dropping lang from either
+    // qk call would collide HU/EN in the cache even while the queryFn still fetches the right language.
+    plexDiscover.mockResolvedValue(fullPayload);
+    plexSurprise.mockResolvedValue({ scope: "both", items: [] });
+    const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    render(
+      <QueryClientProvider client={qc}>
+        <PlexDiscover
+          scope="both"
+          onOpenCard={noop}
+          onInfoCard={noop}
+          onGenre={noop}
+          onSeeAllRecent={noop}
+        />
+      </QueryClientProvider>,
+    );
+    await screen.findByText("plex.discover.continueWatching");
+    const keys = qc
+      .getQueryCache()
+      .getAll()
+      .map((q) => q.queryKey);
+    expect(keys).toContainEqual([
+      "plex-discover",
+      "both",
+      "en",
+      expect.objectContaining({ genres: [] }),
+    ]);
+    expect(keys).toContainEqual([
+      "plex-surprise",
+      "both",
+      "en",
+      expect.objectContaining({ genres: [] }),
+    ]);
   });
 
   it("fires onGenre with the tile's value", async () => {