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/test/queue_panel_test.dart

@@ -6,12 +6,17 @@ import 'package:starsync/src/session/session_coordinator.dart';
 import 'package:starsync/src/shell/library_link.dart' show LinkPreview;
 import 'package:starsync/src/widgets/text_focus.dart';
 
+import 'test_graph.dart';
+
+/// This file's composition root (see test_graph.dart).
+final _graph = testGraph();
+
 void main() {
   testWidgets('Up next popover: holds the keys, Ctrl+V adds with a preview row, + Add link opens a field',
       (tester) async {
     final coord = SessionCoordinator();
     var clip = (files: const <String>[], text: 'look https://example.com/v' as String?);
-    await tester.pumpWidget(MaterialApp(
+    await tester.pumpWidget(scoped(_graph, MaterialApp(
       theme: ThemeData.dark(),
       home: Scaffold(
         body: Center(
@@ -23,7 +28,7 @@ void main() {
           ),
         ),
       ),
-    ));
+    )));
     await tester.pump();
     expect(textInputFocused(), isTrue, reason: 'the player keys rest while it is open');
     expect(find.byKey(const ValueKey('queue-add-field')), findsNothing);