[client] refactor lazy detection (#4050)

This PR introduces a new inactivity package responsible for monitoring peer activity and notifying when peers become inactive.
Introduces a new Signal message type to close the peer connection after the idle timeout is reached.
Periodically checks the last activity of registered peers via a Bind interface.
Notifies via a channel when peers exceed a configurable inactivity threshold.
Default settings
DefaultInactivityThreshold is set to 15 minutes, with a minimum allowed threshold of 1 minute.

Limitations
This inactivity check does not support kernel WireGuard integration. In kernel–user space communication, the user space side will always be responsible for closing the connection.

client/iface/bind/activity_test.go

@@ -0,0 +1,27 @@
+package bind
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+)
+
+func TestActivityRecorder_GetLastActivities(t *testing.T) {
+	peer := "peer1"
+	ar := NewActivityRecorder()
+	ar.UpsertAddress("peer1", netip.MustParseAddrPort("192.168.0.5:51820"))
+	activities := ar.GetLastActivities()
+
+	p, ok := activities[peer]
+	if !ok {
+		t.Fatalf("Expected activity for peer %s, but got none", peer)
+	}
+
+	if p.IsZero() {
+		t.Fatalf("Expected activity for peer %s, but got zero", peer)
+	}
+
+	if p.Before(time.Now().Add(-2 * time.Minute)) {
+		t.Fatalf("Expected activity for peer %s to be recent, but got %v", peer, p)
+	}
+}