feat: add Windows Service integration via --service flag

Adds Microsoft.Extensions.Hosting.WindowsServices package and a --service
CLI flag to Program.cs that logs service mode activation, enabling future
Windows Service lifecycle management.

src/NATS.Server.Host/Program.cs

@@ -3,6 +3,7 @@ using Serilog;
 using Serilog.Sinks.SystemConsole.Themes;
 
 var options = new NatsOptions();
+var windowsService = false;
 
 // Parse ALL CLI flags into NatsOptions first
 for (int i = 0; i < args.Length; i++)
@@ -81,6 +82,9 @@ for (int i = 0; i < args.Length; i++)
         case "--remote_syslog" when i + 1 < args.Length:
             options.RemoteSyslog = args[++i];
             break;
+        case "--service":
+            windowsService = true;
+            break;
     }
 }
 
@@ -133,6 +137,11 @@ else if (options.Syslog)
 
 Log.Logger = logConfig.CreateLogger();
 
+if (windowsService)
+{
+    Log.Information("Windows Service mode requested");
+}
+
 using var loggerFactory = new Serilog.Extensions.Logging.SerilogLoggerFactory(Log.Logger);
 using var server = new NatsServer(options, loggerFactory);