inbox: Sort once, not twice, as List.sort may not be stable

The trick of sorting first by the low-order key and then by
the high-order key works great if the sort algorithm leaves
elements in their existing relative order when they compare
equal, aka if the sort is "stable".

But Dart's List.sort disclaims any guarantee of being stable.
So this trick doesn't work; the pinned streams and unpinned
streams could each get scrambled out of alphabetical order
in the course of the second sort.

Instead do one sort, with a comparison function that looks
at both criteria.

Author: gnprice

lib/widgets/inbox.dart

@@ -123,17 +123,18 @@ class _InboxPageState extends State<InboxPage> with PerAccountStoreAwareStateMix
         && subscriptions.containsKey(entry.key))
       .toList()
       ..sort((a, b) {
+        // TODO "pin" icon on the stream row? dividers in the list?
+        final aPinned = subscriptions[a.key]!.pinToTop;
+        final bPinned = subscriptions[b.key]!.pinToTop;
+        if (aPinned != bPinned) {
+          return aPinned ? -1 : 1;
+        }
+
         final streamA = streams[a.key]!;
         final streamB = streams[b.key]!;
 
         // TODO(i18n) something like JS's String.prototype.localeCompare
         return streamA.name.toLowerCase().compareTo(streamB.name.toLowerCase());
-      })
-      ..sort((a, b) {
-        // TODO "pin" icon on the stream row? dividers in the list?
-        final aPinned = subscriptions[a.key]!.pinToTop;
-        final bPinned = subscriptions[b.key]!.pinToTop;
-        return aPinned == bPinned ? 0 : (aPinned ? -1 : 1);
       });
 
     for (final MapEntry(key: streamId, value: topics) in sortedUnreadStreams) {