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>

app/globals.css

@@ -275,6 +275,17 @@
     }
   }
 
+  @keyframes slideOutRight {
+    from {
+      transform: translateX(0);
+      opacity: 1;
+    }
+    to {
+      transform: translateX(100%);
+      opacity: 0;
+    }
+  }
+
   .animate-fadeIn {
     animation: fadeIn 0.2s ease-out;
   }
@@ -283,6 +294,10 @@
     animation: slideInFromRight 0.3s ease-out;
   }
 
+  .animate-slideOutRight {
+    animation: slideOutRight 0.3s ease-out;
+  }
+
   .animate-slideDown {
     animation: slideDown 0.3s ease-out;
   }