Protocol 11: sit out replaces kicking; nudge

Anyone can sit anyone out (host included); only you bring yourself back.
Out members stay in chat and the party, never block or count, and load no
media; a card with the live position and Join back (Space) replaces the
video. The mini player's x sits you out; optionally idle does too. Nudge:
the server forwards it to the target only (3/s, 20/min per sender); the
target hears the nudge sound, gets a toast and a taskbar attention
request. Kick, its messages, errors and dialog are gone.

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

apps/desktop/windows/runner/flutter_window.h

@@ -3,6 +3,8 @@
 
 #include <flutter/dart_project.h>
 #include <flutter/flutter_view_controller.h>
+#include <flutter/method_channel.h>
+#include <flutter/encodable_value.h>
 
 #include <memory>
 
@@ -28,6 +30,10 @@ class FlutterWindow : public Win32Window {
 
   // The Flutter instance hosted by this window.
   std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
+
+  // "starsync/window": `attention` flashes the taskbar entry while the
+  // window is not in front (lib/src/platform/attention.dart).
+  std::unique_ptr<flutter::MethodChannel<flutter::EncodableValue>> window_channel_;
 };
 
 #endif  // RUNNER_FLUTTER_WINDOW_H_