Send terminal notification on peer session expiry (#1660)

Send notification through terminal on user session expiration in Linux and macOS, 
unless UI application is installed to handle it instead.

client/internal/session.go

@@ -0,0 +1,82 @@
+package internal
+
+import (
+	"context"
+	"os/exec"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/netbirdio/netbird/client/internal/peer"
+)
+
+type SessionWatcher struct {
+	ctx   context.Context
+	mutex sync.Mutex
+
+	peerStatusRecorder *peer.Status
+	watchTicker        *time.Ticker
+
+	sendNotification bool
+	onExpireListener func()
+}
+
+// NewSessionWatcher creates a new instance of SessionWatcher.
+func NewSessionWatcher(ctx context.Context, peerStatusRecorder *peer.Status) *SessionWatcher {
+	s := &SessionWatcher{
+		ctx:                ctx,
+		peerStatusRecorder: peerStatusRecorder,
+		watchTicker:        time.NewTicker(2 * time.Second),
+	}
+	go s.startWatcher()
+	return s
+}
+
+// SetOnExpireListener sets the callback func to be called when the session expires.
+func (s *SessionWatcher) SetOnExpireListener(onExpire func()) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	s.onExpireListener = onExpire
+}
+
+// startWatcher continuously checks if the session requires login and
+// calls the onExpireListener if login is required.
+func (s *SessionWatcher) startWatcher() {
+	for {
+		select {
+		case <-s.ctx.Done():
+			s.watchTicker.Stop()
+			return
+		case <-s.watchTicker.C:
+			managementState := s.peerStatusRecorder.GetManagementState()
+			if managementState.Connected {
+				s.sendNotification = true
+			}
+
+			isLoginRequired := s.peerStatusRecorder.IsLoginRequired()
+			if isLoginRequired && s.sendNotification && s.onExpireListener != nil {
+				s.mutex.Lock()
+				s.onExpireListener()
+				s.sendNotification = false
+				s.mutex.Unlock()
+			}
+		}
+	}
+}
+
+// CheckUIApp checks whether UI application is running.
+func CheckUIApp() bool {
+	cmd := exec.Command("ps", "-ef")
+	output, err := cmd.Output()
+	if err != nil {
+		return false
+	}
+
+	lines := strings.Split(string(output), "\n")
+	for _, line := range lines {
+		if strings.Contains(line, "netbird-ui") && !strings.Contains(line, "grep") {
+			return true
+		}
+	}
+	return false
+}