full-stack-error-handlinglisted

## Purpose Ensure every backend error reaches the frontend UI correctly, with no unhandled error states. ## When to use it Designing or reviewing any feature that involves frontend-backend communication. ## Checklist - **Backend error envelope**: standardize on `{ "error": { "code": "...", "message": "...", "details": [...] } }`. - **Frontend fetch wrapper**: parse the error envelope and throw typed errors, not generic exceptions. - **Component-level handling**: map `error.code` to specific UI treatment — validation errors show field messages, not-found redirects, server errors show retry option. - **No unhandled states**: every data-fetching component has loading, empty, error, and success states. - **Consistency**: same error shape across all endpoints; same error handling pattern across all components. ## Expected output A consistent error propagation chain where every backend failure mode has a corresponding frontend UI treatment.