Add: Reusable client toast notification snippet

[vanosaur]

Adds a lightweight client-side toast notification utility (SnToast) plus a demo client script and README.

[github-actions]

Thank you for your contribution. However, it doesn't comply with our contributing guidelines.

As a reminder, the general requirements (as outlined in the CONTRIBUTING.md file) are the following: follow the folder+subfolder guidelines and include a README.md file explaining what the code snippet does.

Validation errors:

Folder structure violations found:
 - Move 'Client-Side Components/Toast Notification/demo_client_script.js' under a valid folder hierarchy (Category/Subcategory/Use-Case/your-file). Files directly inside 'Client-Side Components' or its subcategories are not allowed.
 - Move 'Client-Side Components/Toast Notification/toast.js' under a valid folder hierarchy (Category/Subcategory/Use-Case/your-file). Files directly inside 'Client-Side Components' or its subcategories are not allowed.

Review your contribution against the guidelines and make the necessary adjustments. Closing this for now. Once you make additional changes, feel free to re-open this Pull Request or create a new one.