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.

backend/app/routes/plex.py

@@ -197,12 +197,6 @@ def watch_import_now(
 # --- Read: libraries / browse / show / image --------------------------------------------------
 
 
-def _enabled() -> None:
-    """Guard read endpoints when the module is off (avoids leaking a stale mirror)."""
-    # Read endpoints stay usable as long as a mirror exists; the toggle only gates sync + UI.
-    return None
-
-
 @router.get("/libraries")
 def list_libraries(user: User = Depends(current_user), db: Session = Depends(get_db)) -> dict:
     """The mirrored libraries the user can browse (drives the Plex scope selector)."""