Modern React UI patterns for loading states, error handling, and data fetching.

• Covers five core principles: never show stale UI, always surface errors, use optimistic updates, progressive disclosure, and graceful degradation
• Provides decision trees and component patterns for loading indicators, skeleton screens, error states, empty states, and button submission handling
• Includes critical anti-patterns to avoid, such as showing spinners when cached data exists, silently swallowing errors, and failing to disable buttons during async operations
• Offers a pre-submission checklist covering UI states, data mutations, and user feedback requirements