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/test/singletons_test.dart

@@ -0,0 +1,47 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+
+/// The app's own `.instance` singletons and how often `lib` reaches for
+/// each. Systems get their dependencies through the composition root
+/// (`AppGraph`, widgets through `AppScope`); this list may only shrink.
+/// Lower a number when a use goes; never raise one or add a class.
+const baseline = {
+  'AppSettings': 55,
+  'JellyfinAccounts': 10,
+  'UpdateController': 8,
+  'Profile': 8,
+  'FolderStore': 6,
+  'AppYtDlp': 4,
+  'RecentLinks': 3,
+};
+
+void main() {
+  test('no new .instance singletons, and no more uses of the old ones', () {
+    final files = Directory('lib')
+        .listSync(recursive: true)
+        .whereType<File>()
+        .where((f) => f.path.endsWith('.dart'))
+        .toList();
+    final sources = [for (final f in files) f.readAsStringSync()];
+    // The app's own classes (framework singletons like
+    // `WidgetsBinding.instance` are not counted).
+    final classes = {
+      for (final s in sources)
+        for (final m in RegExp(r'^(?:abstract |final |base )*class (\w+)', multiLine: true).allMatches(s)) m.group(1)!,
+    };
+    final counts = <String, int>{};
+    for (final s in sources) {
+      for (final m in RegExp(r'\b([A-Z]\w*)\.instance\b').allMatches(s)) {
+        final name = m.group(1)!;
+        if (classes.contains(name)) counts[name] = (counts[name] ?? 0) + 1;
+      }
+    }
+    final grown = {
+      for (final e in counts.entries)
+        if (e.value > (baseline[e.key] ?? 0)) e.key: '${e.value} (baseline ${baseline[e.key] ?? 'none'})',
+    };
+    expect(grown, isEmpty,
+        reason: 'pass the system in through AppGraph / AppScope instead of a new `.instance` use or singleton');
+  });
+}