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/lib/queryKeys.test.ts

@@ -85,14 +85,22 @@ describe("qk — variadic composite plex keys", () => {
     ]);
   });
 
-  it("keys the Discover/Surprise rails by lang so a language switch caches separately", () => {
-    // Both surfaces filter against the language-specific genre column, so HU and EN must not collide.
-    expect(qk.plexDiscover("movie", "hu", FILTERS)).not.toEqual(
-      qk.plexDiscover("movie", "en", FILTERS),
-    );
-    expect(qk.plexSurprise("movie", "hu", FILTERS)).not.toEqual(
-      qk.plexSurprise("movie", "en", FILTERS),
-    );
+  it("preserves scope+lang+filters as distinct Discover/Surprise key segments", () => {
+    // Shape-pinning only: these lock that the variadic factory carries every arg through (a "tidy-up"
+    // that drops a param would break cache separation). That the CALL SITE actually passes lang is
+    // guarded separately in PlexDiscover.dom.test.tsx (it inspects the registered query keys).
+    expect(qk.plexDiscover("movie", "hu", FILTERS)).toEqual([
+      "plex-discover",
+      "movie",
+      "hu",
+      FILTERS,
+    ]);
+    expect(qk.plexSurprise("movie", "hu", FILTERS)).toEqual([
+      "plex-surprise",
+      "movie",
+      "hu",
+      FILTERS,
+    ]);
     // A bare call stays a prefix for the reshuffle/watched-toggle invalidations.
     expect(qk.plexSurprise()).toEqual(["plex-surprise"]);
   });