fix incorrect activeIndex when handling Focus.Previous

This is already available in the React version, now let's sync it in the
Vue version as well.

packages/@headlessui-vue/src/utils/calculate-active-index.ts

@@ -48,6 +48,9 @@ export function calculateActiveIndex<TItem>(
     }
 
     case Focus.Previous: {
+      // If nothing is active, focus the last relevant item
+      if (activeIndex === -1) activeIndex = items.length
+
       for (let i = activeIndex - 1; i >= 0; --i) {
         if (!resolvers.resolveDisabled(items[i], i, items)) {
           return i