feat(ui): implement comprehensive toast notification system

Added a complete toast notification system with:
- Toast store using Zustand for state management
- Toast component with 4 types: success, error, warning, info
- Animated slide-in/slide-out transitions
- Auto-dismiss after configurable duration
- Close button on each toast
- Utility functions for easy access (toast.success(), toast.error(), etc.)

Integrated toast notifications into file operations:
- Success notifications for: open image, open project, export image, save project
- Error notifications for: failed operations
- Warning notifications for: unsupported file types

UI Features:
- Stacks toasts in top-right corner
- Color-coded by type with icons (CheckCircle, AlertCircle, AlertTriangle, Info)
- Accessible with ARIA attributes
- Smooth animations using custom CSS keyframes

This provides immediate user feedback for all major operations throughout
the application.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

store/index.ts

@@ -9,3 +9,4 @@ export * from './transform-store';
 export * from './shape-store';
 export * from './text-store';
 export * from './ui-store';
+export * from './toast-store';