fix: persist saved idea state across navigation, fix UTC timestamp

- Dashboard: track saved ideas in React state (Set), compare against
  suggestion texts to show "✓ Salvata" persistently
- Backend: append 'Z' to saved_at ISO timestamp so JS parses as UTC
  (fixes "2h fa" bug for UTC+2 users)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/content.py

@@ -493,7 +493,7 @@ def save_idea(
         "id": str(uuid.uuid4())[:8],
         "text": text,
         "note": data.get("note", ""),
-        "saved_at": datetime.utcnow().isoformat(),
+        "saved_at": datetime.utcnow().isoformat() + "Z",
         "used": False,
     }
     ideas.insert(0, new_idea)