Add toast notifications for user feedback

- Created ToastContext with showToast hook
- Toast notifications appear in bottom-right, auto-dismiss after 3s
- Added success/error toasts for:
  - Saving notification settings
  - Refreshing prices
  - Updating check interval
  - Deleting products
- Replaced alert() calls with toast notifications

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/vite.config.js

@@ -0,0 +1,14 @@
+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+export default defineConfig({
+    plugins: [react()],
+    server: {
+        port: 5173,
+        proxy: {
+            '/api': {
+                target: 'http://localhost:3001',
+                changeOrigin: true,
+            },
+        },
+    },
+});