Restructure step 3: your server, Invite, version gate, AppGraph

- One server per app run (loopback, no password); Invite attaches an
  all-interfaces listener to the same server and Stop sharing detaches
  it (guests told with 4016); members, media and Up next never reset.
  No solo mode: you're home on your own server unless in a friend's.
- Navigation: none / yours / friend; a join passes a version gate on
  /health first (clear "they/you need to update", update offer).
- Invite as a value (host, port, name, password flag, version, fp
  reserved); addresses filtered (loopback, link-local, 198.18/15,
  bridges) and ranked; Sharing chip, Copy invite, Stop sharing.
- Alone looks like a plain player (alone replaces solo).
- AppGraph builds and disposes systems; widgets use AppScope;
  SessionState deleted; a test caps the remaining singletons.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/shell/mark_problem.dart

@@ -3,7 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:sync_relay_core/core.dart' show RelayConstants;
 
 import '../session/app_events.dart' show ProblemMarked;
-import '../state/session_state.dart';
+import '../app/app_graph.dart';
 import '../widgets/app_menu.dart' show AppGlass;
 import 'motion.dart';
 
@@ -17,8 +17,8 @@ const markProblemKey = SingleActivator(LogicalKeyboardKey.keyB, control: true, s
 /// Asks for the note, then marks; the toast says where it went.
 Future<void> showMarkProblem(BuildContext context) async {
   final note = await showPopover<String>(context, at: Alignment.center, child: const MarkProblemCard());
-  if (note == null) return;
-  final coord = SessionState.instance.coordinator;
+  if (note == null || !context.mounted) return;
+  final coord = AppScope.of(context).coordinator;
   final inSession = await coord.markProblem(note);
   coord.notify(ProblemMarked(inSession: inSession));
 }