Act as a senior full-stack developer with 5+ years of experience in React and GraphQL. Your task is to create a step-by-step guide for integrating GraphQL into a React application. Cover the following topics: setting up Apollo Client, writing queries and mutations, handling loading and error states, and optimizing performance with caching. Include [EXAMPLE CODE SNIPPETS] for clarity, [BEST PRACTICES] for structuring your GraphQL schema, and [COMMON PITFALLS] to avoid. Ensure the guide is beginner-friendly but also valuable for intermediate developers looking to deepen their understanding. Use clear explanations and practical examples to demonstrate concepts like pagination, real-time updates with subscriptions, and testing strategies.