Reafactored: Add Settings Validation and Input Field Enhancements

[dev-alamin]

Summary

This PR adds comprehensive validation for all plugin settings and improves the user experience on the settings page by slightly enlarging input fields.

Changes Introduced

New Method: validate_settings()

• Added a dedicated method to validate all fields before saving.
• Checks required fields:
• Client ID
• Client Secret
• Sanitizes optional fields:
• Whitelisted domains
• Registration enabled checkbox
• One Tap login and screen selection
• Prevents saving empty or invalid values and displays proper admin notices using add_settings_error().

Integrated with register_setting():

register_setting(
    'wp_google_login',
    'wp_google_login_settings',
    [
        'sanitize_callback' => [ $this, 'validate_settings' ],
        'default'           => [],
    ] 
);

• Input Field Enhancement
• Added regular-text class to relevant input fields to make them slightly larger and improve readability.

Benefits

• Ensures required fields cannot be left empty, preventing misconfiguration.
• Improves UX on the settings page with more user-friendly input sizes.
• Ready for future additions — new fields can be validated easily through validate_settings().

Testing

• Manual testing done on a local setup:
• Verified empty Client ID / Secret triggers admin error notice.
• Verified optional fields save correctly.
• Input field sizes updated as expected.

Notes

• On next release: remember to add version number to this method: validate_settings()
• Remember to regenerate/update .pot file as new error text added.
• This PR does not include changes to composer/vendor or node_modules.
• Only includes plugin PHP files and POT updates for new strings.
• Fully backward-compatible with existing settings.