Error/401 handling with toasts

## Acceptance Criteria
- 401 errors → automatic logout → redirect to login
- Network errors → user-friendly toast messages
- Form validation errors → inline error display
- Success actions → confirmation toasts

## Test Notes
- Bad tokens trigger logout flow
- Network failures show helpful messages
- Form validation works correctly
- Toast notifications are properly styled and timed

No files, tests, or entry points are named. Start by locating the React authentication, network-error, form-validation, and toast handling entry points, then run the relevant existing tests if available. Done means 401 responses log out and redirect, network and success cases show styled timed toasts, and validation errors appear inline.

Written by the indexing model from the issue text.