fix: Restore toast notification styles.

[karinakbataeva]

Related Issue

Closes #1210

Summary

Restores the missing toast notification styles that were accidentally removed from the stylesheet, fixing the appearance and dismissal behavior of in-app toast notifications.

Changes Made

• Restored the missing toast notification CSS in css/index.css.
• Restored toast animations (toastSlideIn and toastSlideOut).
• Restored styling for toast variants (success, error, warning, and info).
• Fixed the in-app toast UI so notifications are displayed and dismissed correctly.

Testing

• Started the application locally using npm start.
• Completed a Focus Session to trigger the notification.
• Verified that:
  • Toast notifications are properly styled.
  • The close (×) button dismisses the toast.
  • Toasts auto-dismiss after a few seconds.
  • Only one toast appears after a completed Focus Session.

Screenshots

attached screenshot

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)