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/i18n/locale/en/chatlist.json

@@ -76,6 +76,9 @@
       },
       "waiting_since_desc": {
         "TEXT": "Pending Response: Shortest first"
+      },
+      "priority_desc_created_at_asc": {
+        "TEXT": "Priority: Highest first, Created: Oldest first"
       }
     },
     "ATTACHMENTS": {