Add error toast notifications

[RossCabrera]

Overview

Implement error toast notifications throughout the app to provide users
with clear, actionable error messages from API responses.

Toast System Components

• Toast provider (wrapper)
• useToast() hook for components
• Toast display (top-right, auto-dismiss, etc.)
• Different variants: success, error, warning, info

Error Cases to Handle

• Network errors (offline)
• Server errors (5xx)
• Client errors (4xx)
• Validation errors (from API)
• Timeout errors
• Authentication errors

Implementation Library

• react-hot-toast, sonner, or react-toastify
• Customizable styling
• Accessible (ARIA announcements)

Acceptance Criteria

• Toast notifications display
• Error messages from API shown
• Success messages shown
• Toasts auto-dismiss (e.g., 5 seconds)
• Manual dismiss available
• Accessible (screen readers)
• Don't block user interactions
• Styling matches design

Related Issues

• Depends on: Phase 3, 4