Replace native browser alerts with toast notifications

[haniigupta]

Description

This PR replaces the native browser alert() popups shown after successful login and registration with toast notifications using react-hot-toast.

Closes #168

Changes Made

• Added react-hot-toast and configured the Toaster component.
• Replaced the success alert on the Login page with a toast notification.
• Replaced the success alert on the Register page with a toast notification.
• Kept the existing authentication flow and redirect behavior unchanged.

Testing

• Verified that a toast notification is displayed after a successful login.
• Verified that a toast notification is displayed after a successful registration.

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