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/player_status_test.dart

@@ -1,3 +1,5 @@
+// restructure: rewrite in step 2 (Status + Notifier): the "status" group
+// (one computed level); the title block and overflow checks stay.
 import 'dart:typed_data';
 
 import 'package:flutter/material.dart';
@@ -165,16 +167,6 @@ void main() {
       expect(find.byKey(const ValueKey('now-playing-episode')), findsNothing);
       expect(tester.takeException(), isNull);
     });
-
-    testWidgets('a new media fades through', (tester) async {
-      await tester.pumpWidget(host(const NowPlayingBlock(now: now)));
-      await tester.pumpWidget(host(const NowPlayingBlock(now: NowPlaying(title: 'Dune'))));
-      await tester.pump(const Duration(milliseconds: 60));
-      expect(find.text('Frieren'), findsOneWidget, reason: 'still fading out');
-      await tester.pumpAndSettle();
-      expect(find.text('Frieren'), findsNothing);
-      expect(find.text('Dune'), findsOneWidget);
-    });
   });
 
   testWidgets('start banner: up before playback, fades on play, never back for that media', (tester) async {