Restructure step 0: test safety net, AppEvent/Notifier seam, platform ports, protocol 16 planned

- Tests: the four known failures fixed (test-side causes), one duplicate
  file and four detail-pinning cases removed, every test that pins
  behaviour the restructure changes tagged with its step.
- Every toast, sound and activity-log line goes through Notifier.notify
  (AppEvent), mapped one-to-one to today's behaviour.
- Platform ports (shell, URLs, folders, link resolver, system
  notifications) with desktop implementations; window_manager, Process
  and file_picker now live only in platform/desktop/.
- ROADMAP: protocol 16 planned; app id dev.foxyz.starsync.

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

apps/desktop/test/session_logic_test.dart

@@ -1,3 +1,6 @@
+// restructure: rewrite in step 1 (Connection + Identity + Presence):
+// IdleTracker and unreadyForNewMedia give way to auto sit-out;
+// linkDisplayName stays.
 import 'package:flutter_test/flutter_test.dart';
 import 'package:watch_together/src/session/idle.dart';
 import 'package:watch_together/src/session/session_coordinator.dart';