desktop: document the reaction-rollback race as an accepted edge

Sami flagged in review that onError rolls back to the onMutate snapshot,
so a real event backfilled between onMutate and onError is dropped by the
rollback. It needs a relay error racing an in-flight backfill and
self-heals on the next backfill — vanishingly rare, not worth guarding.
Comment-only, documents the known edge.

Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>

desktop/src/features/messages/hooks.ts

@@ -591,6 +591,10 @@ export function useToggleReactionMutation() {
     },
     onError: (_error, _variables, context) => {
       if (context) {
+        // Rollback restores the pre-click snapshot. Narrow accepted edge: if a
+        // backfill landed a real event between onMutate and onError, this drops
+        // it too — but that needs a relay error racing an in-flight backfill,
+        // and the next backfill self-heals it. Not worth guarding.
         queryClient.setQueryData<RelayEvent[]>(
           context.queryKey,
           context.previous,