fix(plex): round-2 deep-review follow-ups on the review fixes

Second review round, scoped to the round-1 fix commit (where round-1's own fixes
introduced new code):

- The collection chip-remove pruned collectionTitles to an empty {} on last removal,
  which still hashes differently from the pristine no-collection state (key absent) in
  the React Query keys → a residual cache split. Collapse an empty map to undefined so
  the key-shape matches the pristine one.
- Replace the tautological "order-independent signature" test (both objects spread from
  EMPTY_PLEX_FILTERS inherited the same key order, so it proved nothing) with full
  literals in genuinely different insertion order that would fail an order-sensitive
  JSON.stringify signature.
- Cover the collectionTitles chip-remove prune (usePlexActiveFilters.dom.test.tsx) and
  the plexSurprise/plexDiscover lang-key split (queryKeys.test.ts) with regression tests.

Gate: tsc/lint/knip/prettier clean, 412 vitest.

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

@@ -18,7 +18,12 @@ const plexState = {
   show: "watched",
   // A non-default sort is present in the shared state — the hook must IGNORE it (no sort chip).
   sort: { key: "year", dir: "asc" },
-  filters: { ...EMPTY_PLEX_FILTERS, genres: ["Action"] },
+  filters: {
+    ...EMPTY_PLEX_FILTERS,
+    genres: ["Action"],
+    collections: ["A"],
+    collectionTitles: { A: "Marvel" },
+  },
 };
 const setShow = vi.fn();
 const setFilters = vi.fn();
@@ -40,4 +45,21 @@ describe("usePlexActiveFilters", () => {
     expect(keys).not.toContain("sort");
     expect(result.current.every((c) => !c.key.startsWith("sort"))).toBe(true);
   });
+
+  it("removing the last collection chip prunes its title and collapses collectionTitles to undefined", () => {
+    setFilters.mockClear();
+    const { result } = renderHook(() => usePlexActiveFilters());
+    const chip = result.current.find((c) => c.key === "collection:A");
+    expect(chip).toBeDefined();
+    chip!.remove();
+    // patch() merges into the full filters object; assert the two touched fields directly. No residual
+    // { A: "Marvel" } and no empty {} (which would still split the query-key cache from the pristine
+    // no-collection state) — collectionTitles must be undefined.
+    const arg = setFilters.mock.calls[0]![0] as {
+      collections: string[];
+      collectionTitles?: unknown;
+    };
+    expect(arg.collections).toEqual([]);
+    expect(arg.collectionTitles).toBeUndefined();
+  });
 });

frontend/src/components/usePlexActiveFilters.ts

@@ -123,11 +123,13 @@ export function buildPlexActiveFilters(
       label: filters.collectionTitles?.[c] ?? t("plex.filter.collection"),
       // Prune the title too — collectionTitles is hashed into the plex query keys, so a stale entry
       // splits the cache from the logically-identical no-collection state (mirrors CollectionPicker).
+      // Collapse an empty map back to undefined so the last-removed state key-matches the pristine
+      // one (an empty `{}` still hashes differently from an absent key).
       remove: () => {
         const { [c]: _drop, ...restTitles } = filters.collectionTitles ?? {};
         patch({
           collections: filters.collections.filter((x) => x !== c),
-          collectionTitles: restTitles,
+          collectionTitles: Object.keys(restTitles).length ? restTitles : undefined,
         });
       },
     });