feat: add toast notification system (#825)

[CHY9213]

Toast Notification System

A complete toast notification system for bounty status changes, errors, and success messages.

Features

• 4 variants: success (green), error (red), warning (amber), info (blue)
• Auto-dismiss after 5 seconds (configurable)
• Slide-in animation from top-right
• Stack multiple toasts
• Manual close button on each toast
• Accessible with role="alert"
• Dark mode compatible
• Hook-based API for easy usage

Usage

// Wrap your app
import { ToastProvider } from '../contexts/ToastContext';

function App() {
  return (
    <ToastProvider>
      <YourApp />
    </ToastProvider>
  );
}

// Use anywhere
import { useToast } from '../contexts/ToastContext';

function YourComponent() {
  const toast = useToast();
  
  return (
    <button onClick={() => toast.success('Bounty created!')}>
      Create
    </button>
  );
}

Files

• frontend/src/contexts/ToastContext.tsx — Context, Provider, and hook
• frontend/src/components/bounty/ToastContainer.tsx — Visual component with animations
• frontend/src/contexts/toast.ts — Re-export

Closes #825

Wallet: 2oWEBzKDQo9KYxAFYMPrR8QiwKJia4vbSXrbV4ep7eTz