Solo watching and a clearer offline start

Putting anything up offline starts a solo session: an in-app server bound
to 127.0.0.1 only, joined like hosting, so tracking, next episode and the
player all work. Invite friends re-hosts on the Host card's port and
password with the same media and position; Stop needs no confirmation.
Offline, the Library opens with a welcome band (Join a friend, Host, Watch
solo); without a Jellyfin account it leads with Files and Link, Jellyfin
is an optional card. Play buttons work offline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/server/lib/src/serve.dart

@@ -18,7 +18,8 @@ import 'sync_server.dart';
 /// a file on this machine the hosted file (the hosting app's in-session file
 /// swap; a following `swap` with its name and size puts it up) and answers
 /// `{'ok':true,'name':n,'size':s}` or `{'ok':false}`. Without [events] a
-/// start failure throws.
+/// start failure throws. `'loopbackOnly': true` in [config] (next to the
+/// [AppConfig] keys) binds 127.0.0.1 only: the app's solo session.
 Future<void> serve(Map<String, Object?> config, {SendPort? events}) async {
   StreamSubscription<pkg_logging.LogRecord>? logs;
   if (events != null) {
@@ -32,7 +33,7 @@ Future<void> serve(Map<String, Object?> config, {SendPort? events}) async {
   }
   final SyncServer server;
   try {
-    server = SyncServer(AppConfig.fromMap(config));
+    server = SyncServer(AppConfig.fromMap(config), loopbackOnly: config['loopbackOnly'] == true);
     await server.start();
   } catch (e) {
     await logs?.cancel();

packages/server/lib/src/sync_server.dart

@@ -48,8 +48,12 @@ class SyncServer {
   /// Upstream connections of the Jellyfin relay; created on first use.
   HttpClient? _relayClient;
 
+  /// Listen on 127.0.0.1 only (the app's solo session), not on every
+  /// IPv4 interface.
+  final bool loopbackOnly;
+
   /// [ytDlp] replaces the server's own (tests).
-  SyncServer(this.config, {YtDlp? ytDlp}) {
+  SyncServer(this.config, {YtDlp? ytDlp, this.loopbackOnly = false}) {
     hub = ServerHub(sessions, config.secret,
         hostKey: config.hostKey,
         jellyfin: config.jellyfin,
@@ -131,7 +135,7 @@ class SyncServer {
     await _resolveJellyfinServerId();
     _mountRoutes();
     _server = await _app.serve(
-      address: InternetAddress.anyIPv4,
+      address: loopbackOnly ? InternetAddress.loopbackIPv4 : InternetAddress.anyIPv4,
       port: config.port,
       // A killed isolate never closes its listening socket, so the port stays
       // bound in this process. Shared lets the next server in the same