refactor(plex): unify Plex sort onto SortControl + a single SortState (R13 S1)

Plex sort was a bespoke chip-pill row plus a separate asc/desc chip pair, with
the key in PlexState.sort and the direction hidden in PlexFilters.sortDir. Now:
- sort is one SortState {key,dir}; sortDir is dropped from PlexFilters (and from
  the facet-key destructure). The wire call still splits it into sort + sort_dir
  at the server boundary, so the backend is untouched.
- The chip sort-row is replaced by the shared SortControl (select + arrow) — the
  same idiom the feed and channel manager use. New lib/plexSort.ts owns the
  scope-dependent vocabulary (movies add duration) + the title/asc fallback.
- PlexProvider migrates the old storage in place: a legacy bare-key plexSort
  string plus the old filters.sortDir become the {key,dir} state, so an existing
  sort preference survives the upgrade.
- PlexBrowse and PlexSidebar heal the stored sort against the current scope's
  options, so a movie-only key (duration) left over after switching to shows
  can't ask the server for an unavailable sort.

Gate green: tsc clean, eslint 0 errors, prettier clean, vitest 93.

frontend/src/components/PlexBrowse.tsx

@@ -9,6 +9,8 @@ import {
   type ReactNode,
 } from "react";
 import { qk } from "../lib/queryKeys";
+import { healSort } from "../lib/columnSort";
+import { plexSortOptions, PLEX_SORT_FALLBACK } from "../lib/plexSort";
 import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import {
@@ -90,6 +92,13 @@ export default function PlexBrowse() {
   const { q, scope, show, sort, filters, playlistOpen: openPlaylist } = usePlex();
   const { setScope, setFilters, setPlaylistOpen } = usePlexActions();
   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 sub = useHistorySubview<Sub>({ kind: "grid" });
 
   // Sidebar asked to open a playlist → push it as a subview (browser Back returns to the grid), then
@@ -141,14 +150,14 @@ export default function PlexBrowse() {
   }, [sub]);
 
   const browseQ = useInfiniteQuery({
-    queryKey: qk.plexLibrary(scope, dq, sort, show, filters),
+    queryKey: qk.plexLibrary(scope, dq, sortState, show, filters),
     enabled: !!scope && sub.view.kind === "grid",
     initialPageParam: 0,
     queryFn: ({ pageParam }) =>
       api.plexLibrary({
         scope,
         q: dq || undefined,
-        sort,
+        sort: sortState,
         show,
         filters,
         offset: pageParam as number,