Restructure step 2: Status as one computed level, the Notifier's real policy, system notifications

- Status: computed from connection, presence, hold, relay, player facts;
  no setters (health writes and HealthStatus gone); pill, strip and
  source line read it; transitions go to the Notifier.
- Notifier: noticeFor(event, context) with self presence, alone, window
  activity, playing and prefs; toasts for joins/leaves/returns/swaps/
  Up next adds (with picture)/problems/chat, the rest logged; while
  sitting out only chat, nudges, starting, Up next (+ joins if chosen);
  alone no social notices; replayed lines logged; owns its toast timer.
- System notifications (flutter_local_notifications: D-Bus on Linux,
  WinRT on Windows) when the window isn't focused; a click focuses it.
- Toasts sit above the control bar and move with it.
- Versions: each member's in the members card and avatar tooltip,
  the server's in the card header; the strip no longer shows it.

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

apps/desktop/lib/src/shell/session_player.dart

@@ -63,6 +63,9 @@ class SessionPlayer extends StatefulWidget {
   /// Window buttons in the fullscreen top bar (off in tests: they need
   /// window_manager).
   final bool windowControls;
+  /// Follows whether the full player's controls are up (the shell lifts
+  /// its toasts above them); the player keeps its own when null.
+  final ValueNotifier<bool>? controlsShown;
   const SessionPlayer({
     super.key,
     required this.mode,
@@ -79,6 +82,7 @@ class SessionPlayer extends StatefulWidget {
     this.unread = 0,
     this.videoBuilder,
     this.windowControls = true,
+    this.controlsShown,
   });
 
   @override
@@ -96,7 +100,8 @@ class _SessionPlayerState extends State<SessionPlayer> {
   int _shaderNoticeSeen = 0;
   bool _miniHover = false;
   // Whether the controls are up (their frame reports it).
-  final _controlsShown = ValueNotifier(true);
+  final _ownShown = ValueNotifier(true);
+  ValueNotifier<bool> get _controlsShown => widget.controlsShown ?? _ownShown;
   // The mouse on a floating element, or the Up next popover open, keeps
   // the controls up.
   final _hold = ControlsHold();
@@ -155,7 +160,7 @@ class _SessionPlayerState extends State<SessionPlayer> {
     _stopShaderWatch();
     _stopPopoverWatch();
     _noFocus.dispose();
-    _controlsShown.dispose();
+    _ownShown.dispose();
     _hold.dispose();
     _model.dispose();
     super.dispose();