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/test_graph.dart

@@ -0,0 +1,14 @@
+// Test support (not a test): the app's composition root for tests.
+import 'package:flutter/widgets.dart';
+import 'package:starsync/src/app/app_graph.dart';
+import 'package:starsync/src/screens/home/health_probe.dart';
+import 'package:starsync/src/session/identity.dart' show ClientIdentity;
+
+/// One graph per test file, like the app's one per run: nothing starts
+/// until a test calls `navigation.start()` (or `goHome`), so widget tests
+/// see the app before your server is up.
+AppGraph testGraph({HealthGetter healthGet = defaultHealthGet}) =>
+    AppGraph.build(identity: ClientIdentity.ephemeral(), healthGet: healthGet);
+
+/// [child] under [graph]'s [AppScope].
+Widget scoped(AppGraph graph, Widget child) => AppScope(graph: graph, child: child);