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/i18n/locales/en/plex.json

@@ -45,6 +45,7 @@
   },
   "sort": {
     "added": "Recently added",
+    "lastwatched": "Last watched",
     "title": "Title",
     "year": "Year",
     "rating": "IMDb rating",

frontend/src/i18n/locales/hu/plex.json

@@ -45,6 +45,7 @@
   },
   "sort": {
     "added": "Nemrég hozzáadott",
+    "lastwatched": "Utoljára nézve",
     "title": "Cím",
     "year": "Év",
     "rating": "IMDb pont",