Add maximize support and some other stuff

lib/widgets/chat_page/chat_box.dart

@@ -45,6 +45,7 @@ class ChatBox extends HookConsumerWidget {
     }
 
     void send() {
+      if (controller.value.text.isEmpty) return;
       ref
           .watch(RoomChatController.provider(room).notifier)
           .send(

lib/widgets/chat_page/mention_overlay.dart

@@ -22,6 +22,8 @@ class MentionOverlay extends ConsumerWidget {
     super.key,
   });
 
+  // TODO: Don't embed mentions
+
   @override
   Widget build(BuildContext context, WidgetRef ref) => Padding(
     padding: EdgeInsets.all(8),