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/session/connection.dart

@@ -142,7 +142,8 @@ final class ConnReconnecting extends ConnState {
 }
 
 /// The server refused this app; nothing retries. [code]: `version` or
-/// `secret` when the server said so, else empty (offline, timeout,
+/// `secret` when the server said so, `closed` when the host stopped
+/// sharing (close 4016), else empty (offline, timeout,
 /// anything else on the first dial); [message]: in plain words
 /// ([connectErrorText]).
 final class ConnRefused extends ConnState {
@@ -166,6 +167,10 @@ final class ConnReplaced extends ConnState {
 /// [ConnReplaced] in plain words.
 const replacedText = 'Starsync was opened in another window';
 
+/// `ConnRefused('closed')`: the host closed their server to friends
+/// (close 4016).
+const stoppedSharingText = 'the host stopped sharing their server';
+
 /// A version refusal in plain words: [err] is the server's `err`
 /// `version` (its range in `min`/`max`), or a proto 15 server's codeless
 /// `proto mismatch`, which is always an older host.
@@ -518,6 +523,8 @@ class Connection {
         _end(ConnRefused('version', versionText(null)));
       case CloseCodes.secret:
         _end(const ConnRefused('secret', 'wrong server password (401)'));
+      case CloseCodes.closed:
+        _end(const ConnRefused('closed', stoppedSharingText));
       default:
         _dropped(now());
     }