feat(plugins): complete Campaign LA5 cross-host hosting

src/AcDream.Plugin.Abstractions/IPluginHost.cs

@@ -7,6 +7,14 @@ namespace AcDream.Plugin.Abstractions;
 /// </summary>
 public interface IPluginHost
 {
+    /// <summary>
+    /// <see langword="true"/> when <see cref="Ui"/> registrations can be
+    /// projected by this host. No-window hosts return <see langword="false"/>
+    /// and expose <see cref="NoOpUiRegistry.Instance"/> so a plugin may keep
+    /// one code path while deliberately omitting presentation work.
+    /// </summary>
+    bool HasUi { get; }
+
     IPluginLogger Log { get; }
     IGameState State { get; }
     IEvents Events { get; }