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/text.go

@@ -21,3 +21,10 @@ func TextFromFunction(fn func() string) View {
 
 	return View{textNode}
 }
+
+// Span creates a span element with the given text content.
+func Span(text string) View {
+	v := View{element.NewElement("span")}
+	v.e.Set("textContent", text)
+	return v
+}