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/input.go

@@ -205,4 +205,23 @@ func NumberInput(value *reactive.Signal[float64], placeholder string) View {
 	})
 
 	return View{input}
-}
+}
+// RawCheckbox creates a plain checkbox input without a label wrapper.
+// The onChange callback is called with the new checked state when the user clicks.
+func RawCheckbox(checked bool, onChange func(bool)) View {
+	checkbox := element.NewElement("input")
+	checkbox.Attr("type", "checkbox")
+
+	// Set initial state
+	if checked {
+		checkbox.Set("checked", true)
+	}
+
+	// Call onChange when user clicks
+	checkbox.On("change", func() {
+		newValue := checkbox.Get("checked").Bool()
+		onChange(newValue)
+	})
+
+	return View{checkbox}
+}