fix: store StreamSubscription refs so SIGINT/SIGTERM are not GC'd in server mode

Fix: In _waitForSignal(), ProcessSignal.sigint.watch().listen() and
sigterm.watch().listen() returned StreamSubscriptions that were not
stored.  Dart GC collects unowned StreamSubscriptions, which
automatically cancels them, causing Ctrl+C to silently stop working
after an unpredictable interval.

Store both subscriptions and cancel them explicitly after the
completer fires so signal delivery is reliable for the entire
lifetime of the headless server.

Author: Khip01

lib/src/main.dart

@@ -56,11 +56,13 @@ Future<void> main(List<String> args) async {
 
 Future<void> _waitForSignal() async {
   final completer = Completer<void>();
-  ProcessSignal.sigint.watch().listen((_) {
+  final sigintSub = ProcessSignal.sigint.watch().listen((_) {
     if (!completer.isCompleted) completer.complete();
   });
-  ProcessSignal.sigterm.watch().listen((_) {
+  final sigtermSub = ProcessSignal.sigterm.watch().listen((_) {
     if (!completer.isCompleted) completer.complete();
   });
-  return completer.future;
+  await completer.future;
+  await sigintSub.cancel();
+  await sigtermSub.cancel();
 }