feat(plex): 'last watched' browse sort + auto-apply in the in-progress view (S3)

Add a 'lastwatched' sort key to the unified Plex browse. Movies rank by
coalesce(progress_updated_at, watched_at); shows by max(coalesce(...)) across
their episodes (rollup.resumed_at, now broadened from progress-only so watched
episodes count too). Never-watched titles sink via nullslast.

Frontend auto-applies lastwatched:desc when entering the in-progress watch-state
view and restores the pre-entry sort on exit; a manual sort change while there
still wins. i18n HU+EN. Backend tests cover both union legs + mixed nullslast.

frontend/src/lib/plexSort.ts

@@ -4,8 +4,16 @@ import { healSort, type SortOption, type SortState } from "./columnSort";
 // SortControl. Direction lives in the SortState (no more sibling `sortDir` on PlexFilters); the wire
 // call (`plexLibrary`) still splits it into `sort` + `sort_dir` at the server boundary.
 
-const MOVIE_SORTS = ["added", "release", "year", "rating", "duration", "title"] as const;
-const SHOW_SORTS = ["added", "release", "year", "rating", "title"] as const;
+const MOVIE_SORTS = [
+  "added",
+  "lastwatched",
+  "release",
+  "year",
+  "rating",
+  "duration",
+  "title",
+] as const;
+const SHOW_SORTS = ["added", "lastwatched", "release", "year", "rating", "title"] as const;
 
 /** The sort keys a scope offers — movie libraries add `duration`; shows/both don't. */
 function plexSortKeys(scope: string): string[] {