configurable short reminder shown at post creation

### Problem I have

Mostly, I post something on the Fediverse when I'm sad/angry/frustrated about negative situations.
To remind me to (like a thankfulness exercise) that I can make a difference by highlighting positive things on the Fediverse, I would like to have a configurable reminder at the status creation dialogue

### Solution I'd like

The reminder should be shown before I start to type my content. It should not be intrusive or blocking, but still remarkable

The settings should allow to
- define a short text of your own _(can also be used as a reminder to add alt-text to images, not to insult, not to publish OSINT relevant data, to keep it short and simple, etc. ...)_
- activate and deactivate the feature _(default off)_

### Alternatives considered

_No response_

### Other

_No response_