fix(tasks): eliminate duplicate task entries by stabilizing WS listener and deduplicating task state

frontend/src/App.tsx

@@ -208,7 +208,7 @@ export const App: React.FC = () => {
     } else {
       disconnectWebSocket();
     }
-  }, [user]);
+  }, [user?.id]);
 
   // Refresh stats on task/doc real-time events
   useEffect(() => {
@@ -253,7 +253,10 @@ export const App: React.FC = () => {
     target: 'inbox' | 'task' | 'list_item' | 'document' = 'inbox'
   ) => {
     if (target === 'inbox' && thought.status === 'inbox') {
-      setDeck((prev) => [thought, ...prev]);
+      setDeck((prev) => {
+        if (prev.some((t) => t.id === thought.id)) return prev;
+        return [thought, ...prev];
+      });
       setStats((prev) => ({ ...prev, inboxCount: prev.inboxCount + 1 }));
     } else if (target === 'document' && thought.convertedRefId) {
       setSelectedDocId(thought.convertedRefId);