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/pubspec.yaml

@@ -39,6 +39,9 @@ dependencies:
   # App sounds (sfx/, platform/audio/): SoLoud + miniaudio, built from
   # source by its build hook (needs only a C++ toolchain).
   flutter_soloud: ^5.2.1
+  # System notifications (platform/desktop/desktop_notifications.dart):
+  # Linux over D-Bus in pure Dart, Windows toasts through WinRT.
+  flutter_local_notifications: ^22.3.0
 # The sounds are WAVs made in memory: no Ogg/Opus/FLAC codecs, so the
 # build hook downloads and bundles no Xiph libraries.
 hooks: