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/main.dart

@@ -52,8 +52,12 @@ Future<void> main(List<String> args) async {
   final ports = SessionState.instance.ports;
   await ports.shell.start(
       title: dirs.profile == null ? appName : '$appName [${dirs.profile}]', onCloseRequest: shutDownApp);
-  // Focus, minimise and restore feed auto sit-out.
-  ports.shell.watchActivity(SessionState.instance.coordinator.windowActivity);
+  // Focus, minimise and restore feed auto sit-out and the Notifier (in
+  // the background, notices go out as system notifications).
+  ports.shell.watchActivity((a) {
+    SessionState.instance.coordinator.windowActivity(a);
+    SessionState.instance.notifier.window = a;
+  });
   try {
     await SessionState.instance.loadForm();
   } catch (e, st) {