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.d.ts

@@ -0,0 +1,2 @@
+declare const _default: import("vite").UserConfig;
+export default _default;