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/library_rows_test.dart

@@ -226,7 +226,7 @@ void main() {
       final refused = await previewLink('https://cdn.example/b.mp4',
           probe: (url) async => LinkProbe.page(Uri.parse(url), ytDlpError: 'odd', status: 403));
       expect((refused!.title, refused.problem), ('b.mp4', 'Not allowed (403)'));
-      expect(await previewLink('not a link'), isNull);
+      expect(await previewLink('not a link', probe: (url) async => throw StateError('no probe')), isNull);
     });
 
     test('ago text', () {