Improve Form Validation and User Experience in App.js

[git-ellea]

This PR updates the App.js file to enhance the form submission process and user feedback. Key changes include:

• Email Validation: Added a function to check if the email format is valid before submission.
• Improved Error Handling: Displays specific messages for different submission errors, like client and server issues.
• Dynamic Toast Messages: Shows toast notifications for submission success or errors, making it clearer for users.
• State Management: Streamlined the state handling for the form inputs to ensure better user interaction.

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