do a few more remaining todos

flutter/lib/state/fb_message.dart

@@ -5,10 +5,16 @@ import 'package:simplecloudnotifier/state/interfaces.dart';
 part 'fb_message.g.dart';
 
 class FBMessageLog {
-  //TODO max size, auto clear old
+  static const MAX_SIZE = 512;
 
   static void insert(RemoteMessage msg) {
-    Hive.box<FBMessage>('scn-fb-messages').add(FBMessage.fromRemoteMessage(msg));
+    if (!Hive.isBoxOpen('scn-fb-messages')) return;
+
+    final box = Hive.box<FBMessage>('scn-fb-messages');
+
+    box.add(FBMessage.fromRemoteMessage(msg));
+
+    while (box.length > MAX_SIZE) box.deleteAt(0);
   }
 }