Initial commit: watch-together phases 0-4 + presence hardening, internet-tested

apps/desktop/lib/src/session/health.dart

@@ -0,0 +1,7 @@
+enum HealthSeverity { ok, info, warn, error }
+
+class HealthStatus {
+  final HealthSeverity severity;
+  final String message;
+  const HealthStatus(this.severity, this.message);
+}