[client/ios] Add Auth.Stop() to cancel an in-progress interactive login

The iOS PKCE login runs in the main-app process, decoupled from the network
extension (the extension's client context is torn down on login-required, which
would otherwise kill the WaitToken goroutine before the OAuth callback arrives).
Because it is decoupled, nothing aborted the flow when the user dismissed the
browser without logging in: WaitToken kept its loopback HTTP server bound to the
redirect port until the flow expired, so the next connect stalled trying to bind
the same port.

Make the Auth context cancellable and add Auth.Stop(), which cancels it. Cancelling
unblocks WaitToken, whose deferred server.Shutdown frees the port immediately. This
mirrors how Android's stopEngine() aborts login via the engine context.

NewAuthWithConfig now also derives a cancellable context; its only iOS caller uses
LoginSync (no interactive server), so behaviour is unchanged there.

client/ios/NetBirdSDK/login.go

@@ -36,6 +36,7 @@ type URLOpener interface {
 // Auth can register or login new client
 type Auth struct {
 	ctx     context.Context
+	cancel  context.CancelFunc
 	config  *profilemanager.Config
 	cfgPath string
 }
@@ -51,8 +52,19 @@ func NewAuth(cfgPath string, mgmURL string) (*Auth, error) {
 		return nil, err
 	}
 
+	// Use a cancellable context so Stop() can abort an in-progress interactive
+	// login. The PKCE flow's WaitToken blocks (and keeps its loopback HTTP server
+	// bound to a port) until the OAuth callback arrives or the flow expires;
+	// cancelling the context unblocks WaitToken, which then shuts that server down
+	// and frees the port for the next login attempt. iOS runs login in the main-app
+	// process (decoupled from the network extension), so without this the server
+	// lingers after the user dismisses the browser and the next connect stalls
+	// trying to bind the same port.
+	ctx, cancel := context.WithCancel(context.Background())
+
 	return &Auth{
-		ctx:     context.Background(),
+		ctx:     ctx,
+		cancel:  cancel,
 		config:  cfg,
 		cfgPath: cfgPath,
 	}, nil
@@ -60,12 +72,24 @@ func NewAuth(cfgPath string, mgmURL string) (*Auth, error) {
 
 // NewAuthWithConfig instantiate Auth based on existing config
 func NewAuthWithConfig(ctx context.Context, config *profilemanager.Config) *Auth {
+	ctx, cancel := context.WithCancel(ctx)
 	return &Auth{
 		ctx:    ctx,
+		cancel: cancel,
 		config: config,
 	}
 }
 
+// Stop aborts an in-progress interactive login started via Login/LoginWithDeviceName.
+// It cancels the auth context, which unblocks the PKCE WaitToken and shuts down its
+// loopback HTTP server, freeing the redirect port. Safe to call multiple times and
+// safe to call when no login is running.
+func (a *Auth) Stop() {
+	if a.cancel != nil {
+		a.cancel()
+	}
+}
+
 // SaveConfigIfSSOSupported test the connectivity with the management server by retrieving the server device flow info.
 // If it returns a flow info than save the configuration and return true. If it gets a codes.NotFound, it means that SSO
 // is not supported and returns false without saving the configuration. For other errors return false.