fix(useSingle): use built in select and update mandatory default

Author: johnleider

packages/0/src/composables/useSingle/index.ts

@@ -39,7 +39,7 @@ export function useSingle<
   E extends SingleContext<Z> = SingleContext<Z>,
 > (options?: SingleOptions): E {
   const registry = useSelection<Z, E>(options)
-  const mandatory = options?.mandatory ?? true
+  const mandatory = options?.mandatory ?? false
 
   const selectedId = computed(() => registry.selectedIds.values().next().value)
   const selectedItem = computed(() => registry.selectedItems.value.values().next().value)
@@ -51,7 +51,7 @@ export function useSingle<
     if (!item || item.disabled) return
 
     registry.selectedIds.clear()
-    registry.selectedIds.add(id)
+    registry.select(id)
   }
 
   function unselect (id: ID) {