portals2 thanks for large feedback! I answer to your questions:
1) It might be appropriate to add adaptability of the chat size for different screens as well as the ability to resize frame vertically. I'm not sure it's very priority task.
2) It would be good to support desktop first and then mobile version
3) Uploading to a forum's storage via flagrow/upload is not supported for now, but it's planned and it's priority task
4, 5) Yeah, this option is also planned
6) This is a known bug, a fix in progress
7) The user can see their deleted messages only during the current session, and they will disappear after the page is reloaded. If you mean seeing them forever then it's a good idea. I'll add this to todo list
8) I haven't tried it yet and I don't know why, but I'll figure it out