fix(sidebar): clarify empty state when completed conversations are hidden

src/i18n/messages/en.json

@@ -802,6 +802,7 @@
       "conversationCountUnit": "{count, plural, one {# conversation} other {# conversations}}",
       "emptyFolderHint": "No conversations",
       "noMatchingConversations": "No matching conversations",
+      "noUnfinishedConversations": "No unfinished conversations. Enable \"Show completed\" from the top-right menu.",
       "removeFolderConfirmTitle": "Remove folder from workspace?",
       "removeFolderConfirmDescription": "Remove \"{name}\" from the workspace? Its tabs and terminals will close.",
       "folderHeaderMenu": {