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/components/PlexBrowse.tsx

@@ -211,12 +211,6 @@ export default function PlexBrowse({
     await api.plexSetState(card.id, next).catch(() => {});
     qc.invalidateQueries({ queryKey: ["plex-library"] });
   }
-  async function toggleEpisodeWatched(ep: PlexCard) {
-    const next = ep.status === "watched" ? "new" : "watched";
-    optimisticStatus(ep.id, next);
-    await api.plexSetState(ep.id, next).catch(() => {});
-    qc.invalidateQueries({ queryKey: ["plex-library"] });
-  }
   // Apply a metadata filter (clicked on an info page / show hero / cast) then show the unified grid.
   // Array filters (genres/people/studios) UNION with what's set; scalars replace. Clicking a person
   // (cast/crew) widens to the 'both' scope so their movies AND shows show up together (mixed feed).
@@ -348,7 +342,7 @@ export default function PlexBrowse({
                     ep={ep}
                     withShowTitle
                     onPlay={() => onCard(ep)}
-                    onToggleWatched={() => toggleEpisodeWatched(ep)}
+                    onToggleWatched={() => toggleWatched(ep)}
                   />
                 ))}
               </div>