Remove internal package import from todo example

Replace direct internal/element usage with public ui package
components to ensure examples only use the public API:

- Add ui.RawCheckbox for plain checkbox without label wrapper
- Add ui.Span for span elements with text content
- Add View.TextDecoration modifier for strikethrough styling
- Update todo example to use these public components

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/modifiers.go

@@ -170,3 +170,8 @@ func (v View) PointerEvents(value string) View {
 	v.e.PointerEvents(value)
 	return v
 }
+
+func (v View) TextDecoration(value string) View {
+	v.e.Get("style").Call("setProperty", "text-decoration", value)
+	return v
+}