fix(ui): enhance user input validation and error handling

[luannmoreira]

Description

This pull request improves input validation, error handling, and user feedback in the account setup and sign-up flows. The changes aim to enhance the user experience by providing clearer messages and ensuring stricter input validation.

Changes Made

Validation Enhancements:

    Updated username validation rules:
        Required field.
        Must be 3-32 characters.
        Can only contain lowercase letters, numbers, and specific symbols (-_.@).
        Added descriptive error messages for validation failures.
        
    Improved password validation:
        Required field.
        Must be 5-32 characters.
        Includes detailed error messages for length constraints.

Refactoring:

    Simplified computed property syntax in Setup.vue (e.g., hasSign).
    Adjusted hasErrors() in SignUp.vue to include the privacy policy acceptance check.

UI Updates:

    Updated button states in SignUp.vue:
        disabled state now depends on hasErrors() instead of acceptPrivacyPolicy alone.
        Changed button variant dynamically based on validation state.

Error Messaging:

    Improved setup error messages:
        User-friendly success and error notifications.
        Clearer guidance for troubleshooting issues during account setup.

Testing

Verified username and password validation for valid and invalid inputs.
Checked UI states (button enabled/disabled) based on form validation errors.
Ensured error messages display appropriately during account setup failures.

Fixes

fixes #4380

These changes ensure stricter validation and better user guidance, reducing potential input errors and enhancing overall user satisfaction.