login: add nsec qr-scanning

- Allow scanning of QR codes, and if detects a nsec, will provide it to
  the login prompt.

- If nsec is found, provides option to keep nsec in keychain; default is
  to not store

- User stays logged in until they logout, or app is force-quit if nsec
  is not stored.

damusApp.swift:
  Obtains keypair from the notification generated to allow login.

LoginView.swift:
  New views allowing for adding and logic handling the QR reader in
  QRScanNSECView.swift to enable QR scan for nsec.

QRScanNSECView.swift:
  New view to scan for QR code. The sparkling magnifying glass is enabled
  if the view calling the QR view changes the privKeyFound bound variable.

Tipjar: npub1el277q4kesp8vhs7rq6qkwnhpxfp345u7tnuxykwr67d9wg0wvyslam5n0
Closes: https://github.com/damus-io/damus/issues/1291
Changelog-Added: Add QR scan nsec logins.
Signed-off-by: Jericho Hasselbush <jericho@sal-et-lucem.com>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/damusApp.swift

@@ -32,6 +32,9 @@ struct MainView: View {
                     .onReceive(handle_notify(.login)) { notif in
                         needs_setup = false
                         keypair = get_saved_keypair()
+                        if keypair == nil, let tempkeypair = notif.to_full()?.to_keypair() {
+                            keypair = tempkeypair
+                        }
                     }
             }
         }