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/shell/start_banner.dart

@@ -4,7 +4,7 @@ import 'package:flutter/material.dart';
 import 'package:signals/signals_flutter.dart';
 
 import '../session/now_playing.dart';
-import '../state/session_state.dart';
+import '../app/app_graph.dart';
 import 'motion.dart';
 import 'now_playing_title.dart';
 
@@ -21,8 +21,8 @@ class StartCard extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final banner = artImage(now.bannerHash);
-    final cover = banner == null ? artImage(now.posterHash) : null;
+    final banner = artImage(context, now.bannerHash);
+    final cover = banner == null ? artImage(context, now.posterHash) : null;
     final episode = now.episode;
     return LayoutBuilder(builder: (context, box) {
       final h = box.maxHeight;
@@ -145,10 +145,10 @@ class _StartBannerState extends State<StartBanner> {
 
   @override
   Widget build(BuildContext context) {
-    final coord = SessionState.instance.coordinator;
+    final coord = AppScope.of(context).coordinator;
     return IgnorePointer(
       child: ListenableBuilder(
-        listenable: artChanges(),
+        listenable: artChanges(context),
         builder: (context, _) => SignalBuilder(builder: (context) {
           final r = coord.serverState.value;
           final now = coord.nowPlaying.value;