feat: Add a priority + created at sort for conversations (#13658)

- Add a new conversation sort option "Priority: Highest first, Created:
Oldest first" that sorts by priority descending (urgent > high > medium
> low > none) with created_at ascending as the tiebreaker

app/javascript/dashboard/components/widgets/conversation/ConversationBasicFilter.vue

@@ -86,6 +86,10 @@ const chatSortOptions = computed(() => [
     label: t('CHAT_LIST.SORT_ORDER_ITEMS.priority_asc.TEXT'),
     value: 'priority_asc',
   },
+  {
+    label: t('CHAT_LIST.SORT_ORDER_ITEMS.priority_desc_created_at_asc.TEXT'),
+    value: 'priority_desc_created_at_asc',
+  },
   {
     label: t('CHAT_LIST.SORT_ORDER_ITEMS.waiting_since_asc.TEXT'),
     value: 'waiting_since_asc',