Refine #1739 and fix regression causing nostr crossposts and login to not work

[riccardobl]

This PR

• fixes nostr logins and crossposts: Closes bug report nip46 login shows error "m.ZP.getSigner is not a function" #1741
• and refines Close relay connections after each NWC call #1739 by:
  • moving the close method inside the nostr class
  • adding static method to return the ndk singleton
  • disconnecting from the relay after crosspost on the client

[ekzyis]

Do we need a global instance for remote signing? Can't we always create a new instance and then destroy it with nostr.close()? I think it's confusing to use a global instance and individual instances in our code.

Otherwise looks good but I did not test.

[riccardobl]

Yes, we can use a new instance for remote signing.
The singleton is important only serverside, really.