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';

store/toast-store.ts

@@ -0,0 +1,46 @@
+import { create } from 'zustand';
+
+export type ToastType = 'success' | 'error' | 'warning' | 'info';
+
+export interface Toast {
+  id: string;
+  message: string;
+  type: ToastType;
+  duration?: number;
+}
+
+interface ToastStore {
+  toasts: Toast[];
+  addToast: (message: string, type?: ToastType, duration?: number) => void;
+  removeToast: (id: string) => void;
+  clearToasts: () => void;
+}
+
+export const useToastStore = create<ToastStore>((set) => ({
+  toasts: [],
+
+  addToast: (message: string, type: ToastType = 'info', duration: number = 3000) => {
+    const id = `toast-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`;
+    const toast: Toast = { id, message, type, duration };
+
+    set((state) => ({
+      toasts: [...state.toasts, toast],
+    }));
+
+    // Auto-remove after duration
+    if (duration > 0) {
+      setTimeout(() => {
+        set((state) => ({
+          toasts: state.toasts.filter((t) => t.id !== id),
+        }));
+      }, duration);
+    }
+  },
+
+  removeToast: (id: string) =>
+    set((state) => ({
+      toasts: state.toasts.filter((t) => t.id !== id),
+    })),
+
+  clearToasts: () => set({ toasts: [] }),
+}));