Harden join auth flow and sync project baseline

lib/src/state/app_state.dart

@@ -0,0 +1,14 @@
+import 'package:chat_server/models.dart';
+import 'package:chat_server/objectbox.g.dart';
+
+late final Store store;
+late final Box<ChatUser> userBox;
+late final Box<ChatChannel> channelBox;
+late final Box<ChatMessage> messageBox;
+
+void initializeStore() {
+  store = openStore();
+  userBox = store.box<ChatUser>();
+  channelBox = store.box<ChatChannel>();
+  messageBox = store.box<ChatMessage>();
+}