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/text_focus_test.dart

@@ -2,20 +2,25 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 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() {
   // Regression: the old FocusNode-parent walk never met EditableText, so
   // session shortcuts (Space, arrows, F) fired while typing in chat.
   testWidgets('text field focus is detected, buttons are not', (tester) async {
     final field = FocusNode();
     final button = FocusNode();
-    await tester.pumpWidget(MaterialApp(
+    await tester.pumpWidget(scoped(_graph, MaterialApp(
       home: Scaffold(
         body: Column(children: [
           TextField(focusNode: field),
           TextButton(focusNode: button, onPressed: () {}, child: const Text('b')),
         ]),
       ),
-    ));
+    )));
     expect(textInputFocused(), isFalse);
 
     field.requestFocus();
@@ -33,9 +38,9 @@ void main() {
 
   testWidgets('focus under a KeyboardClaim counts as typing', (tester) async {
     final claimed = FocusNode();
-    await tester.pumpWidget(MaterialApp(
+    await tester.pumpWidget(scoped(_graph, MaterialApp(
       home: KeyboardClaim(child: Focus(focusNode: claimed, child: const SizedBox())),
-    ));
+    )));
     expect(textInputFocused(), isFalse);
     claimed.requestFocus();
     await tester.pump();