chore(plex): drop dead _enabled guard, dedup watch-toggle, remove dead i18n

- plex.py: delete the no-op _enabled() (never wired as a Depends/called; the real
  gate is sysconfig plex_enabled).
- PlexBrowse.tsx: collapse toggleEpisodeWatched into toggleWatched — they were
  byte-identical (both take a PlexCard).
- Delete 6 dead plex.json keys (loadMore, playerSoon, filter.library,
  playlist.up/down/remove) from en/hu/de — verified 0 refs (the live
  playlist.removeShow/removeSeason keys are kept).

Behavior-neutral. tsc green, ruff clean on touched files, localdev boots, Plex renders.

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

@@ -12,7 +12,6 @@
     "release": "Erscheinungsdatum"
   },
   "filter": {
-    "library": "Bibliothek",
     "scope": "Bibliothek",
     "scopeOpt": {
       "both": "Alle",
@@ -67,7 +66,6 @@
   "noMatchesFiltered_other": "Keine Treffer — {{count}} Filter schränken die Ergebnisse zusätzlich ein.",
   "loading": "Wird geladen…",
   "empty": "Noch nichts hier. Starte eine Plex-Synchronisierung auf der Admin-Konfigurationsseite.",
-  "loadMore": "Mehr laden",
   "watched": "Angesehen",
   "inProgress": "Läuft",
   "play": "Abspielen",
@@ -92,7 +90,6 @@
     "markSeasonUnwatched": "Staffel als ungesehen",
     "addShowCollection": "Zur Sammlung"
   },
-  "playerSoon": "Player kommt bald — „{{title}}“",
   "collEditor": {
     "manage": "Sammlungen",
     "title": "Sammlungen — {{title}}",
@@ -218,9 +215,6 @@
     "playAll": "Alle abspielen",
     "delete": "Playlist löschen",
     "empty": "Diese Playlist ist leer. Füge Titel über deren Infoseite hinzu.",
-    "up": "Nach oben",
-    "down": "Nach unten",
-    "remove": "Entfernen",
     "layoutAccordion": "Akkordeon-Ansicht",
     "layoutTree": "Baum-Ansicht",
     "removeShow": "Ganze Serie entfernen",