checkbox: support undefined state

src/lib/Checkbox.svelte

@@ -1,5 +1,5 @@
 <script lang="ts" module>
-	export type CheckboxState = 'indeterminate' | boolean;
+	export type CheckboxState = 'indeterminate' | boolean | undefined;
 </script>
 
 <script lang="ts">
@@ -21,7 +21,7 @@
 	let {
 		name,
 		required = false,
-		value = $bindable(false),
+		value = $bindable(),
 		color = 'contrast',
 		class: classValue,
 		children,
@@ -37,7 +37,7 @@
 	<input
 		type="hidden"
 		{name}
-		value={value.toString()}
+		value={value?.toString() ?? 'false'}
 		use:validate={{
 			valfunc: () => {
 				if (required && value !== true) {