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/lib/src/platform/attention.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+/// The runner's window channel (linux/runner/my_application.cc,
+/// windows/runner/flutter_window.cpp).
+const _window = MethodChannel('starsync/window');
+
+/// Asks the desktop to point at the app's window (taskbar flash, urgency
+/// hint) when it is not focused; nothing when it is. The runner clears it
+/// once the window gets focus. Quietly nothing where the runner lacks it.
+Future<void> requestWindowAttention() async {
+  try {
+    await _window.invokeMethod<void>('attention');
+  } on MissingPluginException {
+    // Tests, other platforms.
+  } catch (e) {
+    debugPrint('window attention: $e');
+  }
+}