[Bug] DM message input does not respect instance's character limit

[graves501]

I've been writing some DMs on pixelfed.social v0.12.6.
On that instance there's a 500 character limit per message.

If you use the Pixelfed Web UI you get some feedback for the character limit, but not in Pixelix.

So in my case I wrote a bunch of messages with more than 500 character and tried to send it.
The message input gets cleared and my message is gone, but obviously not sent because the backend checks for the character limit.

So Pixelix should implement a character limit, respecting the instance's limits.