inbox: Use compareTo rather than subtraction for comparison

The subtraction is fine if there isn't overflow, which should
be impossible for Zulip message IDs.  (They need to fit into
54 bits in order to not confuse the web client.)

Still, compareTo feels a bit more explicit about what we mean.

Author: gnprice

lib/widgets/inbox.dart

@@ -150,7 +150,7 @@ class _InboxPageState extends State<InboxPage> with PerAccountStoreAwareStateMix
       topicItems.sort((a, b) {
         final (_, _, aLastUnreadId) = a;
         final (_, _, bLastUnreadId) = b;
-        return bLastUnreadId - aLastUnreadId;
+        return bLastUnreadId.compareTo(aLastUnreadId);
       });
       sections.add(_StreamSectionData(streamId, countInStream, topicItems));
     }