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>

apps/desktop/lib/src/state/server_isolate.dart

@@ -33,8 +33,10 @@ class ServerIsolate {
 
   /// Spawns the server and waits until it listens. Throws a
   /// [StateError] with the server's message if it cannot start.
+  /// [loopbackOnly]: 127.0.0.1 only (the solo session).
   static Future<ServerIsolate> start(
     AppConfig config, {
+    bool loopbackOnly = false,
     required void Function(String line) onLog,
     required void Function(String error) onCrash,
     Duration timeout = const Duration(seconds: 15),
@@ -83,7 +85,7 @@ class ServerIsolate {
     final Isolate isolate;
     try {
       isolate = await Isolate.spawn(
-          _serverMain, (config.toMap(), events.sendPort),
+          _serverMain, ({...config.toMap(), if (loopbackOnly) 'loopbackOnly': true}, events.sendPort),
           errorsAreFatal: false,
           onError: errors.sendPort,
           onExit: exit.sendPort,