feat: add toast notification system (Bounty #825)

[zp6]

Toast Notification System

Closes #825

Implementation

• Toast component with success, error, warning, info variants
• Auto-dismiss after 5 seconds (configurable)
• Slide-in animation from top-right
• Stack multiple toasts
• Accessible (role="alert", aria-live="polite")
• Manual close option

Usage

import { useToast } from '@/components/ui/toast'

function MyComponent() {
  const { addToast } = useToast()
  return <button onClick={() => addToast('Bounty claimed!', 'success')}>Claim</button>
}

Wrap your app with <ToastProvider> to enable toasts globally.

Acceptance Criteria

• Toast notifications appear for key actions
• Auto-dismiss with manual close option
• Multiple toasts stack properly

Wallet: zp6