Implement Toast Notifications for UX Feedback

Description

Currently, when a user logs in incorrectly or successfully uploads a heavy 3D DICOM file, our feedback mechanisms are limited. To make the app feel more like a modern, polished product, we need a global notification system (Toasts) to display success and error messages cleanly in the corner of the screen without disrupting the layout.

Task

• Install a lightweight notification library like react-toastify or react-hot-toast.
• Set up the global Toast container at the top level of the app (e.g., in App.js).
• Trigger an Error Toast when a user inputs invalid credentials on the Login/Signup pages.
• Trigger a Success Toast when a user successfully uploads a new scan.
• Trigger a Success Toast confirming "Logged out successfully" when the user clicks the navigation logout button.

Acceptance Criteria

• Notifications appear on screen for key actions (login errors, successful uploads).
• Notifications automatically dismiss themselves after 3-5 seconds.
• The UI feels responsive and communicates backend states clearly to the end-user.