toHtml method
Converts this row and its children into HTML.
Uses <div> and <table> tags to ensure consistent rendering
across email clients and browsers.
Implementation
@override
String toHtml() {
final rowStyle = _buildRowStyle();
final columnsHtml = _buildColumnsHtml();
return '''
<div style="$rowStyle">
$columnsHtml
</div>
''';
}