Puter mail (self hosted local mailling)

I was (again) thinking of a feature for self hosted that would allow users to send mails across the local network to other users.
They could send text and files across the local network, and this could work if 2 different users are logged at the same time, or if only one is logged.
Received and sended mails by an user could be stored in a new `.mail` folder in the user's home folder.
The folder could look like this in the inside (i put example usernames and UIDs) :

- /[username]/.mail
- received
- joe3985
- mail-2024-07-11-1016.txt
- attachments
- e74c5cd8-6deb-4329-911f-0b81d923ab3d.png
- fc0f6cb6-94f9-4bdb-8f16-d5a6ac73a33d.log
- sended
- naribuildsstuff
- mail-2024-07-11-1018.txt

Filenames :
Mail : `mail-[year]-[month]-[day]-[hour & minutes].txt`
Attachment : `[UID].[extension]`

Example content from `/[username]/.mail/received/joe3985/mail-2024-07-11-1016.txt` :
```
[FROM: joe3985]
[TO: [username]]
[DATE: 2024 07 11 10:16]

[OBJECT: i love Puter]

[CONTENT:]
You know i really like **Puter**, here a screenshot i made in it : 
I also got a little problem, so here's my log in case you can find the problem with it : 

```
The mails could support markdown, thats why i added "**".

This whole file would only be read by the mail app, and the information would then show in a clean way on the UI, like on Gmail, Outlook, etc...

When sending a mail, the user will see a list of all the users on the server, and can type an username if needed.
In case someone (like admins if they exist with https://github.com/HeyPuter/puter/issues/580) don't want to be in the list, they can disappear from the list with a setting in the mail app, but they will still be able to receive mails by typing their username.