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/home_test.dart

@@ -80,9 +80,11 @@ void main() {
 
   test('health parse: online, junk and protocol mismatch', () {
     final ok = ServerHealth.parse(jsonEncode(
-        {'ok': true, 'name': 'Den', 'proto': RelayConstants.protoVersion, 'viewers': 2}));
+        {'ok': true, 'name': 'Den', 'proto': RelayConstants.protoVersion, 'viewers': 2, 'version': '1.3.0'}));
     expect(ok.online, isTrue);
     expect(ok.name, 'Den');
+    expect(ok.version, '1.3.0');
+    expect(ServerHealth.parse('{"ok":true}').version, '', reason: 'a server too old to say');
     expect(ok.viewers, 2);
     expect(ok.needsUpdate, isFalse);
     expect(ServerHealth.parse('{"ok":true,"proto":-1}').needsUpdate, isTrue);