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/test/relay_e2e_test.dart

@@ -206,7 +206,7 @@ void main() {
     await _poll(() => stateMediaRelay(alice.serverState.peek()!)?.source.isNotEmpty ?? false, 'relay state');
     await Future.delayed(const Duration(seconds: 7));
     expect(alice.playerError.peek(), isEmpty, reason: 'no "could not be opened" while the source is on its way');
-    expect(alice.health.peek().message, contains('waiting for bob'));
+    expect(alice.status.peek().line?.text, 'Waiting for bob to pass the video on');
     final src = RelaySource(Uri.parse('ws://127.0.0.1:${server.boundPort}/r/default/$token/source'),
         RelayMedia.single(FileRelayBytes(File(file))));
     await src.start();