nostr implementation and signer improvement

[riccardobl]

signer abstraction

Signer are abstracted to a common interface ( NostrSigner ) so that they can be initialized once (on login/registration) and then passed around the app without worrying about their nature.
It removed the need to branch and duplicate the code for every signer type ie.

 if(signMethod=="..."){
...
}else if(....){
...
} 

signer challenges

At any time signers can trigger a challenge that will automatically open a modal with the instructions to complete it, following the reactive flow model.
This is used to implement nip46 auth challenge and also to request nsec passphrases.

All the input fields requesting the passphrase for signMethod=="nsec" have been removed, since the signer can request it on its own via the challenge, making the code slimmer.

option to remember the nsec passphrase

small quality of life improvement

improved nostr managment

A new NostrManager and nostr context are used to manage connections lifecycle and a fetch() method that autocloses the subscriptions on eose has been added.
Every event is now verified by NostrManager, so there is no need to call verifyEvent every time anymore.