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/widgets/window_buttons.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 
-import '../state/session_state.dart';
+import '../app/app_graph.dart';
 
 class WindowButtons extends StatefulWidget {
   final FocusNode? focusNode;
@@ -18,7 +18,7 @@ class WindowButtons extends StatefulWidget {
 }
 
 class _WindowButtonsState extends State<WindowButtons> {
-  final _shell = SessionState.instance.ports.shell;
+  late final _shell = AppScope.of(context).ports.shell;
   bool _maximized = false;
   late final void Function() _stopWatch;
 
@@ -53,8 +53,9 @@ class _WindowButtonsState extends State<WindowButtons> {
 
   Future<void> _close() async {
     try {
-      await SessionState.instance.coordinator.leave();
-      await SessionState.instance.stopServer();
+      final graph = AppScope.of(context);
+      await graph.navigation.leave();
+      await graph.yourServer.stop();
     } catch (_) {}
     await _shell.requestClose();
   }
@@ -133,7 +134,7 @@ class WindowDragArea extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final shell = SessionState.instance.ports.shell;
+    final shell = AppScope.of(context).ports.shell;
     return GestureDetector(
       behavior: HitTestBehavior.translucent,
       onPanStart: (_) => shell.startDragging(),