Add user online presence tracking and notification

Introduced interfaces and services for tracking user online status, including IOnlineUserStore, IUserNotificationScopeService, and IUserPresenceReader. Added PresenceHub for real-time presence updates via SignalR. Updated OnlinePingingController to use new services and return online status and last seen. Extended UserDto with IsOnline property. Updated dependency injection and privacy settings enum. Removed obsolete IUserPresenceService.

Govor.Application/Services/UserOnlineStatus/OnlineUserStore.cs

@@ -0,0 +1,26 @@
+using Govor.Application.Interfaces.UserOnlineStatus;
+
+namespace Govor.Application.Services.UserOnlineStatus;
+
+public class OnlineUserStore : IOnlineUserStore
+{
+    public void SetOnlineUser(Guid userId)
+    {
+        throw new NotImplementedException();
+    }
+
+    public void SetOfflineUser(Guid userId)
+    {
+        throw new NotImplementedException();
+    }
+
+    public bool IsOnline(Guid userId)
+    {
+        throw new NotImplementedException();
+    }
+
+    public IReadOnlyCollection<Guid> GetAllOnlineUsers()
+    {
+        throw new NotImplementedException();
+    }
+}