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/lib/src/shell/session_player.dart

@@ -15,6 +15,7 @@ import '../queue/queue_ui.dart';
 import '../playback/media_marks.dart' show SkipKind, SkipOffer, formatClock;
 import '../screens/session/jellyfin_widgets.dart';
 import '../screens/session/player_controls.dart';
+import '../session/app_events.dart' show ShaderMessage;
 import '../session/session_coordinator.dart';
 import '../shaders/shader_presets.dart';
 import '../state/session_state.dart';
@@ -92,8 +93,6 @@ class _SessionPlayerState extends State<SessionPlayer> {
   final _noFocus = FocusNode(skipTraversal: true, canRequestFocus: false);
   DateTime? _lastKeyAt;
   late final void Function() _stopShaderWatch;
-  // Own toast ids, negative: the coordinator counts up from 1.
-  int _toastSeq = 0;
   int _shaderNoticeSeen = 0;
   bool _miniHover = false;
   // Whether the controls are up (their frame reports it).
@@ -134,7 +133,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
       final n = _coord.playback.shaderNotice.value;
       if (n == null || n.id == _shaderNoticeSeen) return;
       _shaderNoticeSeen = n.id;
-      _toast(n.text);
+      _coord.notify(ShaderMessage(n.text));
     });
     _stopPopoverWatch = effect(() {
       _hold.set(#upNextPopover, _coord.queue.popoverOpen.value);
@@ -162,13 +161,6 @@ class _SessionPlayerState extends State<SessionPlayer> {
     super.dispose();
   }
 
-  void _toast(String text) {
-    _coord.toasts.value = [
-      ..._coord.toasts.peek(),
-      ToastItem(id: --_toastSeq, text: text, at: DateTime.now().toUtc()),
-    ];
-  }
-
   void _toggleChat() => _model.toggleChat();
 
   /// The player keys, while the player fills the body and nothing (a