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/queue/queue_controller.dart

@@ -181,7 +181,7 @@ class QueueController {
         );
       case LinkMedia(:final url, :final anyway):
         // Alone, a link plays direct: there is nobody to pass it on to.
-        final relay = coord.solo ? LinkRelay.direct : pick.relay;
+        final relay = coord.alone.peek() ? LinkRelay.direct : pick.relay;
         final linked = await mediaForLink(coord.jellyfin, url);
         final ref = linked.jellyfin;
         if (ref != null) {