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/en/plex.json

@@ -12,7 +12,6 @@
     "release": "Release date"
   },
   "filter": {
-    "library": "Library",
     "scope": "Library",
     "scopeOpt": {
       "both": "All",
@@ -67,7 +66,6 @@
   "noMatchesFiltered_other": "No matches — {{count}} filters are also narrowing the results.",
   "loading": "Loading…",
   "empty": "Nothing here yet. Run a Plex sync from the admin Config page.",
-  "loadMore": "Load more",
   "watched": "Watched",
   "inProgress": "In progress",
   "play": "Play",
@@ -92,7 +90,6 @@
     "markSeasonUnwatched": "Mark season unwatched",
     "addShowCollection": "Add to collection"
   },
-  "playerSoon": "Player coming soon — “{{title}}”",
   "collEditor": {
     "manage": "Collections",
     "title": "Collections — {{title}}",
@@ -218,9 +215,6 @@
     "playAll": "Play all",
     "delete": "Delete playlist",
     "empty": "This playlist is empty. Add titles from their info page.",
-    "up": "Move up",
-    "down": "Move down",
-    "remove": "Remove",
     "layoutAccordion": "Accordion view",
     "layoutTree": "Tree view",
     "removeShow": "Remove whole show",