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/jellyfin_session_test.dart

@@ -1,3 +1,5 @@
+// restructure: rewrite in step 5 (What's next + Watch tracking): the
+// "progress" and "episodes" groups (reporting, auto-advance swapper).
 import 'dart:convert';
 
 import 'package:flutter/gestures.dart';
@@ -364,11 +366,6 @@ void main() {
       expect(media.jellyfin!.subtitle, 'Black Summoner · S1E7');
     });
 
-    test('the floating prompt starts two minutes before the end', () {
-      expect(nextPromptStart(null, const Duration(minutes: 24)), const Duration(minutes: 22));
-      expect(nextPromptStart(null, const Duration(seconds: 90)), Duration.zero);
-    });
-
     test('one auto-advance swapper: the lowest id among members with their own account', () {
       final members = [
         member('m10', 'Ann', jf: {fixtureServerId: true}),