fix: lu badge persistence + remove ? + fix duplicate buttons

- EpisodeCard: change 'Lu ?' → 'Marquer' so the toggle button no longer
  shows a confusing question mark next to completed episodes

- Dashboard: fix episodes reappearing in 'Continuer l'écoute' after
  navigation. handleToggleRead now does an optimistic update on BOTH
  local state AND all TanStack Query cache pages (setQueriesData),
  so the 'lu' state survives page navigation without a server roundtrip.
  On error, cache is invalidated to revert to server truth.

- Dashboard: 'Découvrir' button now links to /add (discover new podcasts)
  instead of /trending — was identical to 'Parcourir les tendances' button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/EpisodeCard.tsx

@@ -139,7 +139,7 @@ export default function EpisodeCard({ episode, onPlay, onDetails, onToggleRead }
                  ? <CheckCircle2 className="w-3.5 h-3.5" />
                  : <Circle className="w-3.5 h-3.5" />
                }
-               <span>{isCompleted ? 'Lu' : 'Lu ?'}</span>
+               <span>{isCompleted ? 'Lu' : 'Marquer'}</span>
              </button>
            )}
         </div>