The next task to make me able to create a new post and it should be written to the server.

Let me refresh your memory. The UI will send the post in the Firebase format, which is visible in the following 
screenshot. I will also show you the Magic table for the posts, just to make the conversion more accurate.
// prompts/images/commit18-firebase-posts-schema.png
// prompts/images/commit18-magic-fakebook-schema.png

We should rely only on the SignalR update. We need to change the reducer for that tiny bit, because the post, what 
Magic sends back contains a timestamp, which is exactly "Invalid Date". Later when we fetch the posts the correct 
time stamp is going to be there. Can you update the reducer for this case too?

The postsLoaded reducer used some code in the utils folder to map the Magic format to the Firebase format. We should 
not ignore that. We also only need to modify brand new posts in the postsUpdated reducer regarding the timestamp.

This should be almost working, but let's not forget, that everything is coming from Magic, so we should instead just 
rewrite the code a bit. The old reducer is:
// src/features/posts/postsSlice.js

The following error occurred when I tried to make my new post:
// prompts/images/commit18-console-error.png
// prompts/images/commit18-magic-error.png

Could you use the local variables instead of local storage too?
