Integrate web server in the app to use the app programatically

[Nekidev]

This would be really useful to:

• Create a browser extension that automates the library tracking when users watch an episode or read a manga chapter.
• Remove the code copy-pasting in OAuth2 login.

This'd require some kind of internal authorization system to prevent malicious apps from accessing and/or manipulating the app's library without the user knowing it.

Also, a setting to set the port of the server would be good because we don't want to use a port that the user may need without them knowing it's being used.