fix(sort): address R13 S1 review findings

- PlexSidebar activeCount now compares the HEALED sort, so an out-of-scope key
  (movie "duration" after switching to shows) no longer inflates the
  active-filter badge while the control + grid show the default.
- PlexProvider strips the legacy `sortDir` from the filters blob, so it stops
  leaking into the facets query key and being re-persisted on every change.
- PlexProvider sort migration guards the fall-through: valid JSON that isn't a
  SortState now falls back instead of using the raw JSON string as the key.
- New shared healPlexSort(sort, scope) is the single heal both PlexBrowse and
  PlexSidebar use (removes the noop-label plexSortOptions call + the duplication).
- Tests: useSortState.dom.test.ts (hydrate/persist/heal round-trip via renderHook)
  and SortControl.dom.test.tsx (options, pick, direction toggle, directionless,
  trailing). vitest 94 -> 105.

Gate green: tsc/eslint(0 err)/prettier/knip clean, vitest 105.

frontend/src/components/PlexBrowse.tsx

@@ -9,8 +9,7 @@ import {
   type ReactNode,
 } from "react";
 import { qk } from "../lib/queryKeys";
-import { healSort } from "../lib/columnSort";
-import { plexSortOptions, PLEX_SORT_FALLBACK } from "../lib/plexSort";
+import { healPlexSort } from "../lib/plexSort";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import {
@@ -94,11 +93,7 @@ export default function PlexBrowse() {
   const dq = useDebounced(q.trim(), 350);
   // The provider holds sort scope-agnostically; heal it against THIS scope's options so a movie-only
   // key (e.g. duration) left over from before a switch to shows doesn't ask for an unavailable sort.
-  const sortState = healSort(
-    sort,
-    plexSortOptions(scope, () => ""),
-    PLEX_SORT_FALLBACK,
-  );
+  const sortState = healPlexSort(sort, scope);
   const sub = useHistorySubview<Sub>({ kind: "grid" });
 
   // Sidebar asked to open a playlist → push it as a subview (browser Back returns to the grid), then