Server on an isolate, dedicated server binary, room -> server (proto 5)

- serve(Map config, events) is the one server entrypoint: the app
  hosts it via Isolate.spawn (crash surfaced on the Host card, logs
  forwarded as [server]); bin/server.dart is the dedicated server,
  compiled into the bundle as watch-together-server. --headless gone.
- Host identity via a per-hosting hostKey in hello, replacing the
  loopback heuristic.
- Rename room -> server across code, files and wire (t:"state",
  /health name); protocol bumped to 5.
- CI compiles the server binary; isolate smoke tests in server and
  desktop suites. Docs updated.

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

apps/desktop/lib/src/screens/home/connect_flow.dart

@@ -39,6 +39,12 @@ class ConnectFlow extends ChangeNotifier {
     if (!_disposed) notifyListeners();
   }
 
+  void showError(ConnectMode mode, CardError error) {
+    errors[mode] = error;
+    notices[mode] = null;
+    _notify();
+  }
+
   void clearError(ConnectMode mode) {
     if (errors[mode] == null && notices[mode] == null) return;
     errors[mode] = null;

apps/desktop/lib/src/screens/home/home_page.dart

@@ -33,6 +33,7 @@ class _HomePageState extends State<HomePage> {
   late final _media =
       TextEditingController(text: _f.linkUrl.isNotEmpty ? _f.linkUrl : _f.mediaPath);
   late final void Function() _stopKickWatch;
+  late final void Function() _stopCrashWatch;
   bool _dialogOpen = false;
   String? _lastKickShown;
 
@@ -49,11 +50,21 @@ class _HomePageState extends State<HomePage> {
       }
       WidgetsBinding.instance.addPostFrameCallback((_) => _showKickedDialog(kick));
     });
+    // Back from a session whose hosted server crashed: say so on the Host card.
+    _stopCrashWatch = effect(() {
+      final crash = SessionState.instance.serverCrash.value;
+      if (crash == null) return;
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        SessionState.instance.serverCrash.value = null;
+        if (mounted) _flow.showError(ConnectMode.host, CardError('Server crashed', crash));
+      });
+    });
   }
 
   @override
   void dispose() {
     _stopKickWatch();
+    _stopCrashWatch();
     _flow.dispose();
     for (final c in [_joinHost, _joinPort, _joinSecret, _hostPort, _hostSecret, _media]) {
       c.dispose();