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/session/notice_prefs.dart

@@ -0,0 +1,29 @@
+// The user's notification choices (Settings → Notifications & sounds), as
+// the Notifier's policy reads them. Pure Dart; [AppSettings] stores them.
+
+/// When chat messages pop up as toasts (they always reach the chat).
+enum ChatPopups {
+  all('All'),
+  paused('Only when paused'),
+  off('Off');
+
+  final String label;
+  const ChatPopups(this.label);
+
+  /// Whether a chat message pops up now ([playing]: the video plays).
+  bool shows({required bool playing}) => switch (this) {
+        ChatPopups.all => true,
+        ChatPopups.paused => !playing,
+        ChatPopups.off => false,
+      };
+}
+
+/// What the user picked: chat pop-ups, and whether joins and leaves also
+/// reach them while they sit out or the window is in the background
+/// ([people]; off by default: then only chat, nudges, "starting now" and
+/// Up next do).
+class NotifyPrefs {
+  final ChatPopups chat;
+  final bool people;
+  const NotifyPrefs({this.chat = ChatPopups.all, this.people = false});
+}