toHtml method
Converts the rich text and all its spans into an HTML representation.
Each FlintTextSpan is rendered as a <span> or <a> tag depending
on whether it has a link (onTap property). The alignment is applied
via inline CSS.
Implementation
@override
String toHtml() {
final style =
'text-align: ${align.toCss()}; line-height: 1.6; display: block;';
return '''
<div style="$style">
${children.map((span) => span.toHtml()).join()}
</div>
''';
}